Continuous hydrolysis process for preparing 2- hydroxy-4- methylthiobutanoic acid or salts thereof

ABSTRACT

A continuous process for the preparation of 2-hydroxy-4-methylthiobutanoic acid or a salt thereof which includes introducing an aqueous mineral acid into a nitrile hydrolysis reactor including a continuous stirred tank reactor and introducing 2-hydroxy-4-methylthiobutanenitrile into the nitrile hydrolysis reactor. 2-hydroxy-4-methylthiobutanenitrile is continually hydrolyzed within the nitrile hydrolysis reactor to produce a nitrile hydrolysis reactor product stream containing 2-hydroxy-4-methylthiobutanamide. The nitrile hydrolysis reactor product stream is continuously introduced into an amide hydrolysis flow reactor. 2-hydroxy-4-methylthiobutanamide is continually hydrolyzed within the amide hydrolysis flow reactor to produce an aqueous hydrolyzate product containing 2-hydroxy-4-methylthiobutanoic acid. 2-hydroxy-4-methylthiobutanoic acid is recovered from the aqueous hydrolyzate product.

BACKGROUND OF THE INVENTION

1. This invention relates to the preparation of2-hydroxy-4-methylthiobutanoic acid or salts thereof and moreparticularly to an improved process for preparing an aqueous productcomprising 2-hydroxy-4-methylthiobutanoic acid.

2. 2-hydroxy-4-methylthiobutanoic acid, commonly referred to as thehydroxy analog of methionine and also known as2-hydroxy-4-methylthiobutyric acid or HMBA, is an analog of theessential amino acid methionine. Methionine analogs such as HMBA areeffective in supplying methionine for nutritional uses, particularly asa poultry feed supplement. To efficiently produce feed supplementscontaining HMBA, the hydrolysis must be sufficiently complete.

3. HMBA has been manufactured by various processes involving hydrolysisof 2-hydroxy-4-methylthiobutanenitrile (also known as HMBN or2-hydroxy-4-methylthiobutyronitrile and hereinafter “HMBN” or“nitrile”). HMBA has been produced as a racemic D,L-mixture byhydrolyzing HMBN with a mineral acid, precipitating the acid residue byaddition of an alkaline earth hydroxide or carbonate, and recovering asalt of HMBA from the aqueous phase by evaporative crystallization, asdescribed, for example, in Blake et al U.S. Pat. No. 2,745,745.

4. British Patent No. 915,193 describes a process for the preparation ofthe calcium salt of HMBA in which HMBN is hydrolyzed to HMBA in acontinuous back-mixed reactor using a dilute sulfuric acid solution, andHMBA is separated from the reaction liquor by extraction with an ether.Because of the use of a continuous back-mixed reaction system, theprocess of the British patent may not achieve complete conversion ofHMBN or amide intermediate to HMBA. The presence of significantunreacted material is undesirable where a liquid HMBA product is to bemade.

5. Recently, HMBA has been commercially produced by hydrolyzing HMBNwith sulfuric acid to form a high quality hydrolyzate containing HMBA,extracting HMBA from the hydrolyzate, and recovering the HMBA from theextract as described by Ruest et al. U.S. Pat. No. 4,524,077. In theprocess, HMBN is mixed with sulfuric acid having a strength of betweenabout 50% and about 70% by weight on an organic-free basis at atemperature of between about 25° C. and about 65° C. To control the rateof reaction, the HMBN is preferably added to the acid over a period ofabout 30 to about 60 minutes. Under the preferred conditions,substantial conversion of the nitrile to2-hydroxy-4-methylthiobutanamide (also known as2-hydroxy-4-methylthiobutyramide and hereinafter “amide”) takes place ina period of between about one-half hour and about one and one-halfhours. Thereafter, the amide is converted to HMBA by further hydrolysisat a temperature within the range of between about 70° C. and 120° C.Final hydrolysis of the amide to the acid is carried out in sulfuricacid having an initial strength of between about 30% and about 50% byweight on a organic-free basis. To provide the preferred acid strength,the acid phase is diluted by adding water before heating the reactionmixture. Under conditions of relatively dilute acid strength andincreased temperature, the amide is converted to the acid within aperiod of approximately one and one-half to three hours. In carrying outthe hydrolysis, approximately one mole of sulfuric acid per mole of theHMBN feed is used, with an acid excess of 0 to 10%, preferably 0 to 5%,providing satisfactory results. Ruest et al. describe a batch processand state that a batch process is preferred to ensure that thehydrolysis reaction is carried substantially to completion. If acontinuous reaction system is utilized, Ruest et al. describe that itshould be designed and operated to assure essentially completeconversion. For example, continuous operation could be implemented in aplug flow tubular reactor or cascaded stirred tank system. A singleback-mixed reactor is described by Ruest et al. as providing adequateconversion only at residence times that would generally be consideredunacceptable for commercial production.

6. Hernandez et al. U.S. Pat. No. 4,912,257 describes a process in whichHMBA is produced by sulfuric acid hydrolysis of HMBN in a single step.HMBN is fed to an acidification vessel where it is mixed with 98%sulfuric acid at an acid/nitrile molar ratio between 0.5 and 2 to form areaction mixture containing 20-50% by weight sulfuric acid. The mixtureis agitated and cooled to 50° C. in a continuous addition loop for 30-60minutes as the reaction mixture is produced batchwise. The reactionmixture is then fed to a hydrolysis reactor and heated to a temperatureof between 60° C. and 140° C. for five minutes to six hours whileapplying a slight vacuum to the reactor. The process described byHernandez et al. is said to produce HMBA by hydrolysis of the acidifiedHMBN solution in a single step unlike the two step hydrolysis processesknown in the art.

7. In order to provide a high quality hydrolyzate product containingmaximum HMBA and minimal nitrile and amide components, high conversionof HMBN and 2-hydroxy-4-methylthiobutyramide to HMBA must be obtained.Batch production of HMBA generally provides high conversion. However,conventional batch processes for producing HMBA have several drawbacks.The productivity of a batch process is limited by batch turnaround time.Additionally, the quality of HMBA hydrolyzate can deviate betweenbatches because reaction conditions can vary as each batch is produced.Filling and emptying of the batch reactor and non-steady stateconditions cause vapor emissions that must be treated before release.The equipment required for the prior art processes is costly. Sulfuricacid and water are mixed in an acid dilution tank to form dilutedsulfuric acid feed. A heat exchanger is required to remove the heat ofdilution that is generated within the tank. The tank, heat exchanger,pump and recirculation loop must be of corrosion resistant construction.

SUMMARY OF THE INVENTION

8. Among the several objects of the present invention are the provisionof an improved process for the preparation of HMBA; the provision ofsuch a process that can be operated in a continuous mode; the provisionof such a process that can be operated with high productivity; theprovision of such a process that can significantly reduce capital andmaintenance costs as compared to conventional processes; the provisionof such a process that affords improved control of reaction conditionsas compared to conventional batch hydrolysis systems; the provision ofsuch a process that reduces the vapor emissions as compared toconventional batch systems; the provision of such a process thateliminates the need for separate sulfuric acid dilution, in particular,the provision of such a process that can be operated using aconcentrated sulfuric acid feed stream without prior dilution; theprovision of such a process that effects essentially complete conversionof HMBN to HMBA; and the provision of such a process that can produceHMBA of consistent quality for use in the preparation of animal feedsupplements.

9. These and other objects are obtained through a process for thepreparation of HMBA or a salt thereof including introducing a mineralacid into a nitrile hydrolysis reactor comprising a continuous stirredtank reactor, and introducing 2-hydroxy-4-methylthiobutanenitrile intothe nitrile hydrolysis reactor. 2-hydroxy-4-methylthiobutanenitrile iscontinually hydrolyzed within the nitrile hydrolysis reactor to producea nitrile hydrolysis reactor product stream containing2-hydroxy-4-methylthiobutanamide. The nitrile hydrolysis reactor productstream is continuously introduced into an amide hydrolysis flow reactor.2-hydroxy-4-methylthiobutanamide is continuously hydrolyzed within theamide hydrolysis flow reactor to produce a finished aqueous hydrolyzateproduct containing 2-hydroxy-4-methylthiobutanoic acid.2-hydroxy-4-methylthiobutanoic acid is recovered from the finishedaqueous hydrolyzate product.

10. In another embodiment of the invention,2-hydroxy-4-methylthiobutanoic acid or a salt thereof is produced by aprocess in which 2-hydroxy-4-methylthiobutanenitrile, concentratedsulfuric acid having a strength of between about 70% by weight and about99% by weight, and water are concurrently introduced into a vessel inwhich 2-hydroxy-4-methylthiobutanenitrile is hydrolyzed.2-hydroxy-4-methylthiobutanenitrile is hydrolyzed within the vessel toproduce an aqueous hydrolysis mixture containing2-hydroxy-4-methylthiobutanamide. 2-hydroxy-4-methylthiobutanamide ishydrolyzed to produce a finished aqueous hydrolyzate product containing2-hydroxy-4-methylthiobutanoic acid. 2-hydroxy-4-methylthiobutanoic acidis recovered from the finished aqueous hydrolyzate product.

11. Yet another embodiment of the present invention is directed to anapparatus for use in a process for the preparation of HMBA. Theapparatus includes a first continuous stirred tank reactor for thecontinuous hydrolysis of 2-hydroxy-4-methylthiobutanenitrile in thepresence of a mineral acid to produce an aqueous hydrolysis mixturecontaining 2-hydroxy-4-methylthiobutanamide. The apparatus also includesan amide hydrolysis flow reactor for the continuous hydrolysis of2-hydroxy-4-methylthiobutanamide with sulfuric acid to produce afinished aqueous hydrolyzate product containing2-hydroxy-4-methylthiobutanoic acid.

12. Another embodiment of the invention is directed to a process for thepreparation of 2-hydroxy-4-methylthiobutanoic acid or a salt thereofthat includes introducing 2-hydroxy-4-methylthiobutanenitrile and anaqueous mineral acid into an aqueous hydrolysis mixture comprising2-hydroxy-4-methylthiobutanamide, mineral acid, and unreacted2-hydroxy-4-methylthiobutanenitrile. The2-hydroxy-4-methylthiobutanenitrile in the aqueous hydrolysis mixture ishydrolyzed in a continuous nitrile hydrolysis reactor comprising aback-mixed reaction zone and a circulation zone in fluid flowcommunication with the back-mixed reaction zone. The circulation zonecomprises a circulating line. The aqueous hydrolysis mixture iscontinuously circulated in a circulating stream that is withdrawn fromthe back-mixed reaction zone, passed through the circulation zone andreturned to the back-mixed reaction zone. The circulating stream aswithdrawn from the back-mixed reaction zone contains unreacted2-hydroxy-4-methylthiobutanenitrile. A portion of the aqueous hydrolysismixture is removed from a forward flow port in the circulation zone toform a nitrile hydrolysis reactor product stream. The nitrile hydrolysisreactor product stream is transferred to an amide hydrolysis flowreactor. The nitrile hydrolysis reactor product stream is diluted withwater at a point downstream of the forward flow port to provide afinishing reaction stream. The 2-hydroxy-4-methylthiobutanamidecontained in the finishing reaction stream is hydrolyzed in the amidehydrolysis flow reactor to produce a finished aqueous hydrolyzateproduct containing 2-hydroxy-4-methylthiobutanoic acid. The sum of theresidence time of the circulating stream in the circulation zoneupstream of the forward flow port and the residence time of the nitrilehydrolysis reactor product stream downstream of the forward flow portprior to dilution is sufficient to substantially extinguish residual2-hydroxy-4-methylthiobutanenitrile prior to the dilution of the nitrilehydrolysis reactor product stream.

BRIEF DESCRIPTION OF THE DRAWINGS

13.FIG. 1 is a schematic flowsheet of the process of the invention,illustrating continuous manufacture of HMBA from HMBN, water and amineral acid;

14.FIG. 2 is a schematic flowsheet of a preferred process of the typeillustrated in FIG. 1 as modified to assure that residual HMBN issubstantially extinguished;

15.FIG. 3 is a schematic flowsheet of a process of the invention inwhich 2-hydroxy-4-methylthiobutanamide exiting a nitrile hydrolysisreactor is converted to HMBA in a continuous stirred tank reactor and anamide hydrolysis flow reactor operated in series;

16.FIG. 4 is a schematic flowsheet of a preferred process of the typeillustrated in FIG. 3 as modified to assure that residual HMBN issubstantially extinguished;

17.FIG. 5 is a schematic illustration of a continuous stirred tankreactor adapted for conversion of HMBN to2-hydroxy-4-methylthiobutanamide while a concentrated sulfuric acidstream is introduced into the reactor;

18.FIG. 6 is a schematic illustration of a continuous stirred tankreactor adapted for the HMBN to amide conversion as shown in FIG. 5while extinguishing HMBN prior to dilution;

19.FIG. 7 is a schematic flowsheet of a bench-scale continuoushydrolysis process in which 2-hydroxy-4-methylthiobutanamide exiting afirst recirculating reactor is converted to HMBA in a secondrecirculating reactor and a plug flow reactor operated in series;

20.FIG. 8 is a plot showing amide concentration, nitrile concentration,and Gardner color for the hydrolyzate product as a function ofacid/nitrile molar ratio fed to the first reactor and temperature withinthe plug flow reactor based on bench scale experiments; and

21.FIG. 9 is a schematic flowsheet of a bench-scale continuoushydrolysis process in which 2-hydroxy-4-methylthiobutanamide exiting afirst reactor is introduced into a plug flow reactor and hydrolyzed toproduce HMBA.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

22. In accordance with the present invention, a process for thepreparation of HMBA is provided in which HMBN is continuously hydrolyzedin an aqueous mineral acid to form 2-hydroxy-4-methylthiobutanamide(hereinafter referred to as “nitrile hydrolysis”), and the amide iscontinuously hydrolyzed to form HMBA (hereinafter “amide hydrolysis”).The process is implemented utilizing an apparatus that comprises a firstcontinuous stirred tank reactor (hereinafter “CSTR”) for nitrilehydrolysis and an amide hydrolysis flow reactor, preferably a plug flowreactor (hereinafter “PFR”), for subsequent amide hydrolysis. Thenitrile hydrolysis is very exothermic and is, therefore, mostefficiently conducted in a CSTR back mixed for heat transfer andtemperature control. The amide hydrolysis is less exothermic yet must bebrought to substantial completion in order to achieve desired productquality and yield. A PFR has been found to be well suited for the amidehydrolysis because it can be configured to operate without substantialback-mixing, yet provide adequate residence time for the reactionwithout requiring excessive pressure drop. For example, it has beenfound that an industrial scale pipeline reactor can be operated at aReynolds number in excess of about 5000 without excessive pressure dropthrough the reactor, while producing a hydrolyzate containing less thanabout 0.1% amide and less than about 0.1% nitrile on an HMBA basis.

23. More particularly, the invention is directed to an apparatusincluding a nitrile hydrolysis reactor comprising a CSTR for receivingaqueous mineral acid and HMBN feed streams. For purposes of the presentinvention, an aqueous mineral acid is comprised of water and up to 99wt. % mineral acid. The aqueous mineral acid is generally sulfuric acidor hydrochloric acid. Sulfuric acid is particularly preferred. As theHMBN reacts with water within the CSTR, an aqueous hydrolysis mixturecontaining 2-hydroxy-4-methylthiobutanamide is formed. The amidetypically hydrolyzes to some extent in the nitrile hydrolysis reactor,resulting in formation of ammonium salts and HMBA in the aqueoushydrolysis mixture. The aqueous hydrolysis mixture is continuouslyremoved from the CSTR, cooled, and returned to the CSTR. A portion ofthe circulating aqueous hydrolysis mixture is removed from a forwardflow port to form a nitrile hydrolysis reactor product stream. Thisstream is diluted to form a finishing reaction stream before beingintroduced to the flow reactor for completion of amide hydrolysis. Sincethe amide hydrolysis proceeds to some degree during the nitrilehydrolysis, it is generally preferable to dilute the nitrile hydrolysisreactor product solution as soon as practicable to provide water foramide hydrolysis and prevent liquid phase separation. Dilution alsoprevents precipitation of ammonium bisulfate when sulfuric acid is used.However, when the point of dilution is closely coupled to the point ofwithdrawal of aqueous hydrolysis solution from the CSTR, it has beenfound that residual nitrile may be introduced into the finishingreaction stream. Since the rate of nitrile hydrolysis is reduced byaddition of dilution water, residual nitrile may be increased in theproduct.

24. It has further been discovered that residual nitrile can be reducedto very low levels by providing a modest but critical flow regimeresidence time for substantial completion of the nitrile hydrolysisreaction before dilution. More particularly, it has been discovered thatresidual nitrile is substantially extinguished by providing a nitrilehydrolysis flow regime residence time of at least about 20 seconds and,depending upon the degree of back mixing, preferably between about 30seconds and about 5 minutes, as constituted, e.g., by the sum of theresidence time of the circulating aqueous hydrolysis solution betweenthe point of withdrawal from the CSTR and a forward flow port plus theresidence time of the nitrile hydrolysis reactor product stream betweenthe forward flow port and the point of dilution. For purposes of thepresent invention, residual nitrile is substantially extinguished whennot more than about 0.05% by weight nitrile remains in the finishingreaction stream.

25. It has also been discovered that concentrated sulfuric acid, water,and HMBN can be concurrently and directly introduced into the first CSTRin order to produce within the CSTR a more dilute effective sulfuricacid strength suitable for hydrolysis of HMBN. HMBN, water andconcentrated sulfuric acid can be simultaneously fed into the first CSTRwithout hindering the hydrolysis of HMBN despite the disparate densityand viscosity of sulfuric acid and HBMN and the high heat of dilutionreleased when sulfuric acid is diluted with water. The dilution ofsulfuric acid within the reactor eliminates the need for separate aciddilution as used in a conventional process, reducing cost andmaintenance of the hydrolysis system. Water and acid required for thehydrolysis reaction can be introduced in any practical combination ofconcentrated acid, dilute acid, and water to achieve the requiredconcentration and proportion of mineral acid for the first hydrolysis.

26. In one embodiment of the invention illustrated in FIG. 3, thenitrile hydrolysis reactor product stream removed from the nitrilehydrolysis reactor is diluted with water to form an amide hydrolysisstream that is fed to a second CSTR before being transferred to theamide hydrolysis flow reactor. Alternatively, the nitrile hydrolysisreactor product stream and the water stream can be introduced directlyto the second CSTR. A substantial portion of the amide is converted toHMBA in the second CSTR by further hydrolysis to form the finishingreaction stream. The finishing reaction stream is further hydrolyzed inan amide hydrolysis flow reactor located downstream of the second CSTRto form a finished aqueous hydrolyzate product containing HMBA.Alternatively, the second CSTR can be bypassed such that the finishingreaction stream is continuously fed directly to the amide hydrolysisflow reactor and hydrolyzed to form the hydrolyzate product. It has beendiscovered that the process of the invention can be operated at highproductivity in one or more CSTRs in series together with a flowfinishing reactor. Thus, the capital costs of implementing the processare significantly reduced as compared to the batch processes previouslyconsidered necessary in the art to provide adequate conversion at highproductivity.

27. It has been found that such a continuous hydrolysis process canprovide efficient conversion of HMBN to HMBA to produce a high qualityhydrolyzate product containing very low amounts of HMBN and2-hydroxy-4-methylthiobutanamide. In order to produce quality feedsupplements containing HMBA, the process of the invention may beoperated at high productivity to produce a finished aqueous hydrolyzateproduct comprising at least about 36 wt. % HMBA, at least about 18 wt. %ammonium salt, at least about 20 wt. % water, up to about 0.05 wt. %amide and up to about 0.05 wt. % nitrile. The HMBA within the finishedaqueous hydrolyzate product includes HMBA monomer as well as dimers andother oligomers. When the mineral acid used in the hydrolysis issulfuric acid, the finished aqueous hydrolyzate product comprises atleast about 36 wt. % HMBA, at least about 30 wt. % ammonium salt, suchas ammonium bisulfate or ammonium sulfate, at least about 25 wt. %water, up to about 0.05 wt. % amide and up to about 0.05 wt. % nitrile.When hydrochloric acid is used in the hydrolysis, the finished aqueoushydrolyzate product comprises at least about 50 wt. % HMBA, at leastabout 18 wt. % ammonium chloride, at least about 20 wt. % water, up toabout 0.05 wt. % amide and up to about 0.05 wt. % nitrile. In aparticularly preferred embodiment of the invention, substantiallycomplete conversion is achieved during start up with sulfuric acid aswell as at steady state so that the preferred hydrolyzate productcomposition may be consistently produced throughout all processoperations.

28. An aqueous hydrolyzate product of lesser purity can also be preparedaccording to the process of the present invention by using a lower acidto nitrile ratio for the hydrolysis. Such an aqueous hydrolyzate productcomprises at least about 30 wt. % 2-hydroxy-4-methylthiobutanoic acid,at least about 20 wt. % ammonium salt, such as ammonium sulfate orammonium bisulfate, at least about 25 wt. % water, up to about 5 wt. %amide and up to about 0.1 wt. % nitrile, and has a color of not morethan about 10 on the Gardner scale.

29. Ordinarily, the hydrolyzate product produced before steady stateconditions are established, for example, during start up, could containmore amide and nitrile than is desired in a high quality HMBA product.It has been discovered that such composition fluctuations can beprevented by operating at a higher mineral acid to nitrile molar ratioduring start up in order to establish steady state conditions veryrapidly. Presumably, all of the mineral acid and HMBN are introducedinto the first CSTR reactor, but the mineral acid stream can be dividedto introduce one portion directly into the amide hydrolysis flowreactor. Broadly speaking, therefore, the mineral acid to nitrile molarratio is based on the cumulative rates at which mineral acid and nitrileare introduced into the process as a whole. Operation at a highermineral acid to nitrile ratio is achieved by controlling the rate ofmineral acid flowing into the amide hydrolysis flow reactor so that itis at least stoichiometrically equivalent to the sum of the nitrile andamide flowing into that reactor. When sulfuric acid is used for thehydrolysis, the sulfuric acid to nitrile molar ratio is between about1.0 and about 2.0 from start up of the process until steady state isestablished, preferably between about 1.0 and about 1.5, and morepreferably between about 1.15 and about 1.25. After steady state isreached, the sulfuric acid to nitrile molar ratio is between about 0.6and about 1.5, preferably between about 0.9 and about 1.2, and morepreferably between about 0.95 and about 1.05. When hydrochloric acid isused for the hydrolysis, the steady state hydrochloric acid to nitrilemolar ratio is between about 1.0 and about 1.5, preferably between about1.05 and about 1.3, and more preferably between about 1.15 and about1.2. The above-described preferred acid to nitrile ratios are optimalfor a high productivity process. For most effective control at highproductivity, the mineral acid rate is preferably at least 5% in excessof the rate equivalent to the sum of nitrile and amide. Decreasing acidto nitrile ratios can reduce color of a finished aqueous hydrolyzateproduct and reduce operating costs. Operation at the lower acid tonitrile ratios described herein can be preferred if low cost, lowproductivity production of a finished aqueous hydrolyzate product isdesired.

30. Referring to FIG. 1, 2-hydroxy-4-methylthiobutanamide iscontinuously generated by the hydrolysis of HMBN in a CSTR 10. At startup of the process, a mineral acid feed stream is introduced into thereactor 10 and mixed in a back-mixed reaction zone 12 therein. HMBN isthen introduced into the mineral acid stream where it reacts with waterto form the amide within the aqueous hydrolysis mixture. Continuousnitrile hydrolysis occurs as the HMBN and mineral acid streams arecontinuously fed to the aqueous hydrolysis mixture within the reactor10.

31. The mineral acid is preferably sulfuric acid having a strength ofbetween about 50% by weight and about 70% by weight, preferably betweenabout 60% by weight and about 70% by weight. Sulfuric acid serves as acatalyst and is not consumed in the nitrile hydrolysis reaction.However, acid is consumed by the amide hydrolysis reaction thatgenerally occurs to some extent in the nitrile hydrolysis reactor,resulting in formation of ammonium bisulfate when sulfuric acid is usedin the nitrile hydrolysis reaction. The reaction is carried out at atemperature between about 40° C. and about 70° C., preferably betweenabout 60° C. and about 65° C., and at a total pressure of between about0 and about 15 psig. The residence time during which the aqueoushydrolysis mixture is contained within the reactor 10 is between about20 minutes and about 60 minutes, preferably between about 25 minutes andabout 45 minutes. The residence time within the CSTR 10 is computed bydividing the volume of the aqueous hydrolysis mixture in the CSTR 10 andin the circulating line 14 by the volumetric flow rate of nitrilehydrolysis reactor product stream transferred downstream via forwardflow port 26. The aqueous hydrolysis mixture produced in the CSTR 10comprises up to about 16 wt. % HMBA, up to about 12 wt. % ammonium salt,at least about 6 wt. % water, at least about 30 wt. % amide and up toabout 2 wt. % nitrile. When sulfuric acid is used, the aqueoushydrolysis mixture produced in the CSTR 10 comprises up to about 16 wt.% HMBA, up to about 12 wt. % ammonium salt, such as ammonium bisulfateor ammonium sulfate, at least about 6 wt. % water, at least about 35 wt.% amide and up to about 2 wt. % nitrile, preferably between about 5 andabout 12 wt. % HMBA, between about 4 and about 9 wt. % ammonium salt,between about 10 and about 15 wt. % water, between about 35 and about 50wt. % amide and up to about 2 wt. % nitrile, and more preferably,comprises between about 5 and about 11 wt. % HMBA, between about 4 andabout 8 wt. % ammonium salt, between about 11 and about 13 wt. % water,between about 40 and about 50 wt. % amide and up to about 1 wt. %nitrile.

32. When hydrochloric acid is selected as the mineral acid for thenitrile hydrolysis reaction, the acid preferably has a strength ofbetween about 30% by weight and about 40% by weight, more preferablybetween about 35% by weight and about 37% by weight. Hydrochloric acidserves as a catalyst and is not consumed in the nitrile hydrolysisreaction. However, hydrochloric acid is consumed by the amide hydrolysisreaction that generally occurs to some extent in the nitrile hydrolysisreactor, resulting in formation of ammonium chloride in the aqueoushydrolysis mixture. Any ammonium chloride solids can be dissolved afterthe finished aqueous hydrolyzate product is obtained. The reaction iscarried out at a temperature between about 25° C. and about 60° C.,preferably between about 45° C. and about 55° C., and at a totalpressure of between about 2 and about 15 psig. The residence time duringwhich the aqueous hydrolysis mixture is contained within the reactor 10is between about 25 minutes and about 60 minutes, preferably betweenabout 40 minutes and about 50 minutes. When hydrochloric acid is used,the aqueous hydrolysis mixture produced in the CSTR 10 comprises up toabout 10 wt. % HMBA, up to about 5 wt. % ammonium chloride, at leastabout 20 wt. % water, at least about 40 wt. % amide and up to about 2wt. % nitrile, preferably between about 2 and about 10 wt. % HMBA,between about 0.5 and about 5 wt. % ammonium chloride, between about 20and about 30 wt. % water, between about 40 and about 60 wt. % amide andup to about 0.5 wt. % nitrile, and more preferably, comprises betweenabout 5 and about 9 wt. % HMBA, between about 0.5 and about 4 wt. %ammonium chloride, between about 25 and about 30 wt. % water, betweenabout 45 and about 60 wt. % amide and up to about 0.1 wt. % nitrile.

33. In an alternative embodiment of the invention, the mineral acidstream can be divided such that a portion is fed to the CSTR 10 and theremainder is fed via line 18 upstream of the inlet of the in-line mixer32 as shown in FIGS. 1, 2 and 4, or via line 20 into the amidehydrolysis mixture fed to a continuous amide hydrolysis reactor, secondCSTR 36, as shown in FIG. 3. The acid to nitrile molar ratio into theCSTR 10 is between about 0.6 and about 1.5 and, preferably between about0.8 and about 1.2. The overall molar acid to nitrile ratio is betweenabout 0.7 and about 1.5, preferably between about 0.9 and about 1.2 and,more preferably, between about 0.95 and about 1.05. The overall molaracid to nitrile ratio, for purposes of the present invention, is the sumof any mineral acid feed streams divided by the nitrile fed to thenitrile hydrolysis reactor.

34. Lower acid to nitrile molar ratios are preferred when low cost, lowproductivity production of a finished aqueous hydrolyzate product isdesired. The acid to nitrile molar ratio into the CSTR 10 under theseconditions is between about 0.5 and about 0.95 and, preferably betweenabout 0.8 and about 0.95. The overall molar acid to nitrile ratio isbetween about 0.6 and about 0.95, and preferably between about 0.85 andabout 0.95.

35. In FIG. 1, the continuous nitrile hydrolysis reactor also includes acirculation zone in fluid flow communication with the back-mixedreaction zone 12. The circulation zone comprises a circulating line 14through which the aqueous hydrolysis mixture withdrawn from theback-mixed reaction zone 12 is continuously circulated via a pump 22. Anexternal heat exchanger 24 is preferably included in the circulatingline to remove exothermic heat of reaction by transfer to a coolant.After passing through the pump 22 and the heat exchanger 24, the aqueoushydrolysis mixture is returned to the back-mixed reaction zone 12. Aportion of the aqueous hydrolysis mixture is removed from a forward flowport 26 in the circulation zone to form a nitrile hydrolysis reactorproduct stream which is transferred to an amide hydrolysis flow reactor16.

36. The temperature of the circulating aqueous hydrolysis mixture is atleast 30° C., preferably between about 40° C. and about 60° C.throughout the circulation zone. When sulfuric acid is used, thetemperature is at least about 50° C., preferably between about 55° C.and about 60° C. throughout the circulation zone, and when hydrochloricacid is used, the temperature is at least about 30° C., preferablybetween about 40° C. and about 50° C. throughout the circulation zone.The reactor 10 can be jacketed to provide additional cooling capacity,and also to provide for heating the contents of the reactor if requiredduring start up.

37. The liquid level within the reactor 10 is maintained constant by alevel controller. Although the liquid level can also be controlled bygravity overflow from the reactor, the hydrolysis system is more easilydesigned if positive level control is utilized. A level controller isalso preferred because the aqueous hydrolysis mixture is viscous.Moreover the availability of a level controller allows the workingvolume and residence time of the reactor be varied at the operator'sselection, e.g., to adapt to changes in throughput.

38. In a particularly preferred embodiment of the invention, a flowregime residence time of at least about 20 seconds is provided toeffectively complete the nitrile hydrolysis before dilution of thereaction solution for carrying out the amide hydrolysis. FIG. 2illustrates such an arrangement. The aqueous hydrolysis mixture leavingthe back-mixed reaction zone 12 is preferably not removed from thesuction side of a pump 22, but is instead transferred through thecirculating line 14 via the pump 22 to a forward flow port 26 located onthe circulating line a distance sufficiently downstream of the pump toprovide a flow reaction regime, preferably a plug flow section in whicha portion of residual nitrile is consumed. Advantageously, the forwardflow port 26 is so located to provide a residence time of at least about3 seconds, generally between about 3 seconds to about 15 seconds, andpreferably between about 5 seconds and about 10 seconds in thecirculating line 14 upstream of port 26 to promote reaction of residualnitrile. Because of the substantial volume of the aqueous hydrolysismixture that is circulated through the heat exchanger 24 for removal ofthe heat of reaction, the flow regime in the circulating line is readilymaintained in a turbulent regime, so that the portion of the circulatingline 14 between the point of withdrawal from CSTR 10 and port 26functions essentially as a plug flow reactor.

39. A portion of the circulating aqueous hydrolysis mixture is removedas a nitrile hydrolysis reactor product stream from forward flow port26, while the remainder of the circulating stream flows back to CSTR 10.In order to assure that residual nitrile is most effectively reduced, aflow regime residence time of at least about 30 seconds, preferablybetween about 30 seconds and about 5 minutes, more preferably betweenabout 30 seconds and about 3 minutes and, more preferably between about2 minutes and about 3 minutes is provided for completion of the nitrilehydrolysis reaction in a flow regime nitrile extinction reaction regionconstituted of the portion of the circulation line between the exit ofthe CSTR 10 and the forward flow port 26 together with the nitrilehydrolysis reactor product transfer line 28 upstream of dilution point30. Since, as discussed above, a residence time of between about 3seconds and about 15 seconds is advantageously provided between thepoint of withdrawal from the CSTR 10 and the port 26, the transfer line28 is preferably configured to provide a residence time of between about10 seconds and about 5 minutes, more preferably between about 30 secondsand about 5 minutes and, most preferably, between about 1 minute andabout 3 minutes. In order to provide the requisite residence timewithout excessive flow velocity and pressure drop, transfer line 28 maybe configured for laminar flow but, whatever the flow conditions, atleast one equivalent back-mixed reaction zone is provided in thistransport line. The transfer line 28 can also be configured forturbulent flow. Preferably, the velocity is such as to provide theequivalent of at least about two equivalent back-mixed reaction zones,more preferably between about 3 and about 5 sequential back mixedreaction zones in the aforesaid nitrile extinction reaction region. In aparticularly preferred embodiment, the residual HMBN in the nitrilehydrolysis reactor product stream at the point of dilution 30 is notgreater than about 0.01 wt. % based on the sum of the HMBA and amidecontained in the nitrile hydrolysis reactor product stream.

40. The transfer line 28 is preferably configured as a verticaldowncomer so that nitrogen or other gases that may become entrained inthe aqueous hydrolyzate mixture in the CSTR 10 can be disengaged fromthe downward flowing liquid and vented through the top of the downcomertransfer line 28. Although a horizontal configuration can be used, anygases could disengage from forward flow in a horizontal line andaccumulate, reducing effective liquid volume in the line.

41. As the nitrile hydrolysis reactor product stream is transferreddownstream of the forward flow port 26, it is mixed with a water streamand any divided portion of the mineral acid flow in an in-line mixer 32to form a finishing reaction stream. The nitrile hydrolysis reactorproduct stream is mixed with the water stream and any acid stream toassure that a homogeneous liquid mixture is introduced to the amidehydrolysis flow reactor 16. Any water stream also further dilutesmineral acid within the finishing reaction stream, provides reactionwater to be consumed during amide hydrolysis, and reduces the viscosityof the finishing reaction stream. When sulfuric acid is used, dilutionof the nitrile hydrolysis reactor product stream may avoid liquid phaseseparation or precipitation of ammonium bisulfate within the amidehydrolysis flow reactor 16. Ammonium chloride will typically precipitatewhen hydrochloric acid is used for hydrolysis. The water stream istypically introduced at a rate that provides sulfuric acid strength inthe finishing reaction stream of between about 30% and about 50% byweight on an organic-free basis, preferably between about 35% to about45% by weight, more preferably about 43% by weight. If about 30% toabout 38% by weight hydrochloric acid is used, water addition is notrequired. The hydrochloric acid strength in the finishing reactionstream is between about 30% and about 40% by weight on an organic-freebasis, preferably between about 35% to about 38% by weight, morepreferably about 36% by weight.

42. The water stream can be heated before being mixed with the nitrilehydrolysis reactor product stream to form the finishing reaction stream,or the finishing reaction stream can be heated before being fed to theamide hydrolysis flow reactor 16 to provide the desired reactoroperating temperature. Typically, the water stream is heated to atemperature of between about 60° C. and about 100° C., preferablybetween about 70° C. and about 90° C., and, more preferably betweenabout 75° C. and about 80° C. If the water is not preheated or thetemperature of the finishing reaction stream is too low, the stream canbe brought to the requisite temperature in a preheater 34. The residencetime of the finishing reaction stream between the point of dilution 30and the inlet to the amide hydrolysis flow reactor is not critical. Thefinishing reaction stream is well mixed within this region and thenenters the amide hydrolysis flow reactor 16.

43. In the amide hydrolysis flow reactor, some of the residual HMBN ishydrolyzed to form additional amide and amide is substantiallyhydrolyzed to form HMBA. Preferably, the molar ratio of water to amidefed to the amide hydrolysis flow reactor is between about 5 and about10. The flow rate of the finishing reaction stream is preferablyoperated to maintain suitable velocity within the amide hydrolysis flowreactor to maintain turbulence and minimize axial back mixing therein.

44. As indicated above, at a given residence time in the amidehydrolysis flow reactor, it has been found that conversion may besubstantially increased by increasing the mineral acid/nitrile molarratio in the feed. Experience has shown that in some instances steadystate is obtained in about two hours during start up when the sulfuricacid/nitrile molar ratio is 1.0, yet steady state conditions, andcomplete conversion, may be obtained almost immediately when thesulfuric acid/nitrile molar ratio is 1.2. Rapid establishment of steadystate conditions enables consistent production of a high qualityhydrolyzate product that comprises up to about 0.05 wt. % amide and upto about 0.05 wt. % nitrile upon leaving the amide hydrolysis flowreactor 16.

45. The expense of the excess mineral acid, however, may be prohibitiveif an increased mineral acid/nitrile molar ratio is maintained duringroutine operation of the process after steady state conditions areestablished. Thus, it is preferable to use a sulfuric acid/nitrile molarratio of between about 1.0 and about 1.5, preferably between about 1.15and about 1.25, only from initial start up until steady state conditionsare established in the amide hydrolysis flow reactor in order to avoidpreparation of off-specification hydrolyzate product during start-up.Such a molar ratio is obtained when the molar excess of sulfuric acidadded to the amide hydrolysis flow reactor 16 is between about 0 andabout 50%, preferably between about 15 and about 25%, over thatstoichiometrically equivalent to the amide and HMBN introduced to theamide hydrolysis flow reactor. After steady state is established, thesulfuric acid/nitrile molar ratio can then be adjusted to, andmaintained at, a more cost effective molar ratio of between about 0.9and about 1.2, preferably between about 0.95 and about 1.05. The waterfeed rate into the mixer 32 may be increased to avoid liquid phaseseparation of organic and aqueous phases when a sulfuric acid/nitrilemolar ratio below 1.0 is used. When hydrochloric acid is used for thehydrolysis, the hydrochloric acid to nitrile molar ratio during steadystate is between about 1.0 and about 1.5, preferably between about 1.05and about 1.3, and more preferably between about 1.1 and about 1.2. Sucha molar ratio is obtained when the molar excess of hydrochloric acidadded to the amide hydrolysis flow reactor 16 is between about 0 andabout 50%, preferably between about 5 and about 30%, more preferablybetween about 10 and about 20%, over that stoichiometrically equivalentto the amide and HMBN introduced to the amide hydrolysis flow reactor.

46. It has been discovered that operation of the amide hydrolysis flowreactor at a high mineral acid/nitrile molar ratio during start upimproves conversion of amide to HMBA within the amide hydrolysis flowreactor 16 without darkening the color of the hydrolyzate product.Despite the increased severity of reaction conditions provided by a highacid/nitrile ratio, it has unexpectedly been discovered thatacid/nitrile molar ratio does not significantly affect hydrolyzatecolor. Moreover, the high acid to nitrile molar ratio also allows amidehydrolysis flow reactor operation at a lower temperature during steadystate operation, therefore producing a light colored hydrolyzateproduct.

47. The hydrolyzate product leaving the amide hydrolysis flow reactor 16has a light color of between about 5 to about 10, preferably betweenabout 5 to about 7, as measured using a Gardner calorimeter. Color isadversely affected by excessive amide hydrolysis flow reactortemperatures and residence time within the amide hydrolysis flow reactor16. The amide hydrolysis flow reactor operates at a temperature betweenabout 70° C. and about 120° C. When the amide hydrolysis flow reactor isoperated adiabatically, the temperature rises along the flow path as thereaction product absorbs the adiabatic heat of reaction, reaching apoint on the flow path (hot spot) at which the temperature reaches aplateau, and beyond which it may drop slightly if conditions are lessthan perfectly adiabatic. The peak temperature in the amide hydrolysisflow reactor is preferably between about 90° C. and about 120° C., morepreferably between about 90° C. and about 105° C. The residence time ofthe finishing reaction stream within the amide hydrolysis flow reactoris between about 30 minutes and about 100 minutes, preferably betweenabout 50 minutes and about 70 minutes. When the amide hydrolysis flowreactor is operated at a temperature above 110° C., a darker hydrolyzatemay be produced. However, an amide hydrolysis flow reactor temperaturebelow 90° C. may result in incomplete amide hydrolysis unless a higheracid to nitrile molar ratio is employed. Darkening of the hydrolyzateproduct can also occur if the residence time exceeds about 120 minutes.A light colored hydrolyzate product is produced when an acid/nitrilemolar ratio of between about 1.1 and about 1.5 is used during start upand normal operation when the amide hydrolysis flow reactor 16 isoperated at a moderate temperature of between about 70 and about 95° C.,preferably between about 80° C. and about 90° C. The amide hydrolysisflow reactor temperature can be reduced when it is operatedadiabatically by lowering the temperature of the water stream enteringthe mixer 32. If the finishing reaction stream is introduced into thepreheater 34 (FIG. 1) before it is introduced to the amide hydrolysisflow reactor, the heat applied to the preheater can be reduced to lowerthe amide hydrolysis flow reactor operating temperature. Alternatively,cooling and/or heating may be provided to control the amide hydrolysisflow reactor temperature when the amide hydrolysis flow reactor isoperated isothermally. When a second CSTR 36 precedes the amidehydrolysis flow reactor 16 as shown in FIG. 3, a darkened hydrolyzateproduct may be produced if the operating temperature of the second CSTRis too high. A light colored hydrolyzate product is produced when theabove described acid/nitrile molar ratio is used and the second CSTR isoperated at a moderate temperature of between about 70 and about 95° C.,preferably between about 80° C. and about 90° C.

48. The flow reactors best suited for use in the amide hydrolysisprocess of the invention are plug flow reactors configured for operationat a Peclet number of at least 50 at a PFR operating temperature of atleast 90° C. The Peclet number (Pe) is a measure of axial back mixingwithin the PFR as defined by the following equation:

Pe=uL/D

49. where: u=velocity, L=length, and D=axial dispersion coefficient. ThePeclet number of a PFR is inversely proportional to axial back-mixing.Axial back mixing is effectively minimized when the Peclet number is atleast 50, preferably between about 50 and about 200 or more, andresidence time is between about 40 and about 100 minutes, preferablybetween about 50 and about 60 minutes.

50. The PFR 16 of the present invention may be a pipeline PFR or apacked column PFR filled with a packing material. The amide hydrolysisreaction is non-zero order, but the kinetics of reaction have been foundsufficiently favorable that high conversion may be realized within therelatively modest residence times noted above, and without substantialpressure drop. More particularly, it has been found that, where thenitrile has been substantially converted to amide, and the nitrileconcentration is not greater than about 2% by weight in the streamentering the plug flow reactor, the residual amide and nitrileconcentrations may each be reduced to not greater than about 0.2% byweight on an HMBA basis in a pipeline reactor that is operated with avelocity of the reacting stream in the turbulent flow range regime, forexample, at a Reynolds number of at least about 3000, preferably atleast about 5000. Provided that the nitrile/amide ratio of the finishingreaction stream entering the reactor is not greater than about 1% byweight in the stream entering the PFR, the amide and nitrileconcentrations in the reaction product may each be reduced to notgreater than about 0.1% by weight, HMBA basis. For the relatively modestresidence time required to achieve such conversion, a PFR reactor can beoperated at turbulent velocity without excessive pressure drop.Moreover, it has been found that the desired conversion may be attainedat a modest operating temperature, in the range of between 90° C. andabout 105° C., which does not require a high pressure reactor, and whichallows the preparation of a product having a light color.

51. Alternatively, a packed column PFR may be used to carry out thefinal hydrolysis reaction. By use of structured packing, a packed columnreactor may be operated at a significantly lower velocity than apipeline reactor without significant back mixing due to wall effects orchanneling. The packing promotes turbulence and radial mixing, andminimizes axial back mixing, dead spots and channelling of flow so thatall fluid elements travel through the PFR in about the same residencetime. Thus, a packed column reactor may have a substantially greaterdiameter and a more compact configuration than a pipeline reactor. It isparticularly advantageous where reactants or products are of highviscosity.

52. However, for the process of the invention, it has been found that apipeline reactor, i.e., an elongate tubular reactor substantially devoidof internal packing or other internal flow obstructions, is preferred.While a slightly greater degree of axial back mixing per unit length maybe incurred in a pipeline reactor, the kinetics of the nitrile and amidehydrolysis have been discovered to allow nearly quantitative conversionwith the modest residence times and low pressure drops described above.Because of low pressure drop incurred even at high velocity in a reactorsuitable for the process of the invention, a pipeline reactor may beconfigured, i.e., with a high L/D (length to diameter) ratio, to operateat a very high Peclet number, typically in excess of 200, and readily inexcess of 2000. Additionally, a pipeline reactor for the process of theinvention can be constructed of relatively inexpensive materials ofconstruction, e.g., teflon-lined carbon steel pipe. For a packed columnreactor, more exotic materials of construction may be required. Apipeline reactor also affords greater flexibility since it can beoperated at a much greater turndown ratio than a packed column, in thelatter of which conversion declines sharply below a well definedthreshold velocity. Threshold velocity in a packed column is attained inthe transition between laminar and turbulent flow.

53. The amide hydrolysis flow reactor 16 is insulated to compensate forheat losses to the atmosphere. The heat of reaction generated during theamide hydrolysis is sufficient for autothermal operation under adiabaticconditions. Advantageously, the finishing reaction stream can enter theamide hydrolysis flow reactor at a temperature below the reactiontemperature for the amide hydrolysis. During autothermal operation, theheat of reaction generated by amide hydrolysis increases the temperaturewithin the amide hydrolysis flow reactor, lessening the likelihood thata hot spot will form therein. The temperature profile in the amidehydrolysis flow reactor can be measured through several temperaturesensors T_(i) (FIGS. 1-4) along the length of the reactor. The waterfeed temperature can be adjusted to achieve the desired temperatureprofile in the amide hydrolysis flow reactor by increasing or decreasingthe heat supplied to the water feed stream by the water heater 38 beforeit enters the mixer 32 to form the finishing reaction stream.Additionally, the temperature of the finishing reaction stream exitingthe mixer 32 can be raised through the use of preheater 34 to increasethe amide hydrolysis flow reactor operating temperature.

54. Although residual nitrile hydrolyzes in the inlet portion of theamide hydrolysis flow reactor, the nitrile hydrolysis should proceedsufficiently to completion in the CSTR 10 and in the nitrile extinctionreaction region comprising the portion of the circulation zone upstreamof the forward flow port and the zone within which the nitrilehydrolysis reactor product stream flows between the forward flow port 26and the point of dilution 30. Heat of reaction from hydrolysis ofsubstantial quantities of nitrile in the amide hydrolysis flow reactorpotentially creates hot spots within the reactor. Although hot spottemperatures of as much as 110° C. to about 120° C. can be toleratedwithin the amide hydrolysis flow reactor, the hydrolyzate product candarken significantly under such conditions. Substantially extinguishingthe nitrile within the nitrile hydrolysis reactor product stream allowsfor operation of the amide hydrolysis flow reactor at a lowertemperature to provide a light colored finished aqueous hydrolyzateproduct.

55. The amide hydrolysis flow reactor operates at a total pressure ofbetween about 0 and about 15 psig. A pressure control valve at theoutlet of the amide hydrolysis flow reactor provides up to 15 psig backpressure to avoid boiling in the reactor system when the amidehydrolysis flow reactor operates at a temperature higher than 105° C.

56. Advantageously, amide hydrolysis samples may be withdrawn fromsample valves S (FIGS. 1-4) and analyzed via gas chromatography todetermine the amide hydrolysis composition profile along the length ofthe amide hydrolysis flow reactor. Once steady state conditions areestablished, a hydrolyzate sample can be removed from the amidehydrolysis flow reactor outlet every eight to twelve hours andquantitatively analyzed to monitor product quality.

57. The finished aqueous hydrolyzate product exiting the amidehydrolysis flow reactor 16 flows through a cooler 40 before being storedin a hydrolyzate product surge tank 42. The nitrile hydrolysis reactor,the amide hydrolysis reactors and the hydrolyzate product surge tankutilized in the processes of the present invention are operated underthe same overhead pressure (preferably about 10 psig) by employing acommon vent header that is blanketed with nitrogen and controlled by apressure controller that relieves pressure by venting gases to anincinerator header when pressure exceeds about 15 psig. Venting mayremove volatile organic sulfur compounds such as methyl sulfide, methyldisulfide, and methyl mercaptan, which are by-products of the reaction.The vapor emissions are less than 0.5 scf per 1000 lbs. HMBA product,usually less than 0.3 scf per 1000 lbs. product. Emissions of 0.2scf/1000 lbs. HMBA and even lower are readily achievable, especiallywhere only a single CSTR is used.

58. HMBA, or a salt or derivative thereof, can be recovered from theaqueous hydrolyzate product for use in making animal feed supplements.For example, the HMBA in the hydrolyzate can be recovered for use in aliquid phase animal feed supplement comprising between about 80% andabout 98% by weight, preferably between about 80% and about 95% byweight, of the total of weight proportions of HMBA, and between about 2%and about 20% by weight, preferably between about 5% and about 20% byweight water, and having a color of not greater than about 8 as measuredon the Gardner scale, a kinematic viscosity at 25° C. as measured by aCannon-Fenske viscometer of not greater than about 500 centistokes,preferably 90 centistokes, and which, upon subjection to acceleratingrate colorimetry exhibits neither exothermic nor endothermicthermochemical effects at any temperature less than about 150° C.

59. The HMBA can be recovered from the finished aqueous hydrolyzateproduct by neutralization with ammonium hydroxide as described byHernandez et al. U.S. Pat. No. 4,912,257, which is incorporated hereinby reference, or by extraction methods such as that described by Ruestet al. U.S. Pat. No. 4,524,077, which is incorporated herein byreference.

60. WO 96/01808, WO 96/01809, and WO 96/05173, which are incorporatedherein by reference, describe methods for preparation of an ammoniumsalt of HMBA, for preparation of concentrated HMBA by sulfuric acidhydrolysis of nitrile, and for recovery of HMBA by thin film evaporationwith solvent recovery, respectively. More particularly, WO 96/01808describes preparation of an ammonium salt by neutralizing the finishedaqueous hydrolyzate product and using solvent extraction to form anorganic phase containing HMBA and an ammonium acid salt aqueous phase.The application states that the organic phase is then treated withammonia to form a second aqueous phase containing an ammonium salt ofHMBA and an organic solvent phase, and the ammonium salt of HMBA isrecovered from the second aqueous phase. Ammonia is said to be recoveredby treating the ammonium acid salt solution with sodium hydroxide toform sodium chloride or sodium sulfate which may be more easily disposedof. WO 96/01809 describes formation of concentrated HMBA by extracting afinished aqueous sulfuric acid hydrolyzate product with an organicsolvent to form an HMBA-containing organic phase and an aqueous phase,and evaporating the HMBA-containing organic phase to provideconcentrated 98% HMBA containing less than 4 wt. % water. WO 96/05173describes recovery of HMBA by evaporating a finished aqueous sulfuricacid hydrolyzate product to obtain a practically water-free,HMBA-containing salt residue, treating the residue with an organicsolvent to form a suspension, separating the solids from the suspensionto form an HMBA-containing solution, removing the organic solvent fromthe HMBA-containing solution to obtain an HMBA residue, and adding waterto the HMBA residue to form an HMBA solution.

61. The prior art includes various other methods for recovery of an HMBAor HMBA salt product, including at least several which have beenpracticed commercially. Whatever process may be selected by one skilledin the art for recovery of HMBA product from a hydrolyzate, advantagesaccrue from initial preparation of the aqueous hydrolyzate productaccording to the processes of the present invention. The hydrolyzateproduced in accordance with the invention is highly suited for use inany operative process for recovery of the product acid or salt.

62. Salts of HMBA can also be prepared from the finished aqueoushydrolyzate product by methods described by Cummins et al, U.S. Pat. No.4,310,690, Nufer U.S. Pat. No. 3,272,860, and Blake et al, U.S. Pat.Nos. 2,938,053 and 2,745,745, which are incorporated herein byreference. Cummins describes preparation of a calcium salt of HMBA bymixing an aqueous hydrolyzate containing HMBA and ammonium chloride witha mixture of sodium chloride and a calcium salt, reacting the mixturewith sodium hydroxide, reacting the resulting solution with a calciumhydroxide slurry to form the calcium salt of HMBA, and separating thecalcium salt of HMBA. Blake et al. describe formation of ammonium andcalcium salts of HMBA by neutralizing sulfuric acid in an aqueoushydrolyzate containing HMBA and ammonium sulfate by adding calciumhydroxide, filtering the mixture to remove calcium sulfate, reacting thefiltrate with a calcium hydroxide slurry, filtering the mixture toremove calcium sulfate, and drying the filtrate to recover a compositioncontaining a calcium salt of HMBA, an ammonium salt of HMBA, and smallamounts of calcium sulfate and water. Blake et al. also describepreparation of the calcium salt of HMBA by reacting calcium carbonatewith an aqueous hydrolyzate containing HMBA, sulfuric acid, and ammoniumsulfate to form the ammonium salt of HMBA and calcium sulfate,separating the calcium sulfate, reacting the remaining liquid withcalcium hydroxide to form the calcium salt of HMBA and ammoniumhydroxide, heating the mixture to decompose ammonium hydroxide and driveoff ammonia, filtering the resulting mixture to remove calcium sulfateand calcium hydroxide, and evaporating water from the mixture to form aconcentrated slurry of the calcium salt of HMBA, filtering the slurryand drying the filter cake to obtain the calcium salt of HMBA. Nuferdescribes formation of a calcium salt of HMBA by mixing HMBA with amonoalkyl ether of ethylene glycol, reacting the mixture with a calciumoxide-ethylene glycol ether slurry, filtering the resulting slurry, anddrying the filter cake to recover the calcium salt of HMBA. Othermethods of forming salts or derivatives of HMBA are well known, andinclude methods of preparing salts of HMBA by direct reaction of a metaloxide or other base with isolated or partially isolated HMBA asdescribed in U.S. Pat. Nos. 4,855,495, 4,579,962 and 4,335,257, whichare incorporated herein by reference.

63.FIG. 3 illustrates an embodiment of the present invention wherein theamide hydrolysis reaction is conducted in the amide hydrolysis flowreactor 16 and a second CSTR 36 upstream of the amide hydrolysis flowreactor. The second CSTR enables easy handling of the viscous amide,thoroughly mixes the amide hydrolysis mixture with the dilution waterand any divided portion of the mineral acid stream within the secondCSTR and controls the temperature of the finishing reaction stream,resulting in a relatively low viscosity of the latter stream asintroduced into the amide hydrolysis flow reactor. The nitrilehydrolysis reaction takes place in CSTR 10 and the nitrile hydrolysisreactor product stream exiting the CSTR 10, a water feed stream, and anydivided portion of a mineral acid stream via line 20 are introduced intothe second CSTR 36 wherein a substantial portion of the amide ishydrolyzed to HMBA. For purposes of the present invention, a substantialportion of the amide is hydrolyzed when more than 50% by weight,preferably between about 50% and about 80% by weight, of the amide ishydrolyzed to HMBA. The residence time during which the amide hydrolysismixture is contained within the second CSTR 36 is between about 30minutes and about 80 minutes, preferably between about 40 minutes andabout 60 minutes. The residence time within the second CSTR 36 iscomputed by dividing the liquid volume of the second CSTR by thevolumetric flow rate of the finishing reaction stream exiting the secondCSTR. The liquid level in the second CSTR can be controlled by gravityoverflow to the amide hydrolysis flow reactor 16 or preferably bypositive level control as previously described.

64. The amide hydrolysis reaction is initiated in the second CSTR at atemperature between about 70° C. and about 120° C., preferably betweenabout 90° C. and about 105° C., and at a total pressure of between about0 and about 15 psig. Conversion to HMBA is generally improved byoperating the second CSTR at an elevated temperature between about 90°C. and about 110° C. The second CSTR 36 is typically provided with asteam heated jacket in order to maintain the operating temperature. Ifthe temperature sensors T_(i) (FIG. 3) detect a hot spot within theamide hydrolysis flow reactor, the operating temperature of the secondCSTR can be lowered.

65. The amide hydrolysis reaction is substantially carried out in thesecond CSTR, producing a finishing reaction stream that is introducedinto the amide hydrolysis flow reactor 16. The finishing reaction streamcomprises at least about 30 wt. % HMBA, at least about 17 wt. % ammoniumsalt, at least about 15 wt. % water, up to about 10 wt. % amide and upto about 1 wt. % nitrile. Preferably, the finishing reaction streamcomprises between about 30 and about 50 wt. % HMBA, between about 17 andabout 30 wt. % ammonium salt, between about 15 and about 30 wt. % water,between about 1 and about 6 wt. % amide and up to about 0.1 wt. %nitrile. When sulfuric acid is used, the finishing reaction streamcomprises at least about 31 wt. % HMBA, at least about 20 wt. % ammoniumsalt, such as ammonium bisulfate or ammonium sulfate, at least about 20wt. % water, up to about 5 wt. % amide and up to about 1 wt. % nitrile,and preferably between about 32 and about 42 wt. % HMBA, between about20 and about 30 wt. % ammonium salt, between about 22 and about 30 wt. %water, between about 2 and about 4 wt. % amide and up to about 0.1 wt. %nitrile. When hydrochloric acid is used, the finishing reaction streamcomprises at least about 45 wt. % HMBA, at least about 17 wt. % ammoniumchloride, at least about 15 wt. % water, up to about 8 wt. % amide andup to about 1 wt. % nitrile and, preferably between about 45 and about50 wt. % HMBA, between about 17 and about 19 wt. % ammonium chloride,between about 18 and about 22 wt. % water, between about 2 and about 6wt. % amide and up to about 0.1 wt. % nitrile. The amide hydrolysis isthen completed within the amide hydrolysis flow reactor as describedabove for FIG. 1.

66.FIG. 4 is a preferred, modified process of the process shown in FIG.3. Residence time of the nitrile hydrolysis reactor product stream isprolonged between the hydrolysis reactor outlet and the second CSTR toextinguish residual nitrile as described above regarding FIG. 2. Whenthe nitrile hydrolysis reactor product stream is diluted with waterand/or any divided portion of the mineral acid, an amide hydrolysisstream is formed which is fed to the second CSTR. Amide hydrolysisoccurs in the second CSTR to produce a finishing reaction stream to betransferred to the amide hydrolysis flow reactor.

67.FIGS. 5 and 6 illustrate a preferred embodiment of the inventionwherein the CSTR 10 can be adapted in the processes shown in FIGS. 1through 4 to receive concentrated sulfuric acid, HMBN, and water feedstreams. The HMBN and water feed stream are fed directly to the CSTR.The concentrated sulfuric acid stream is either mixed with thecirculating stream in the circulating line or is fed directly to theCSTR. The concentrated sulfuric acid stream may be fed directly to CSTR10 but is preferably fed via line 44 to the circulating line downstreamof the heat exchanger 24 so that the concentrated acid stream isthoroughly mixed with the aqueous hydrolysis mixture before it isreturned to the CSTR 10. When all streams are simultaneously fed to theCSTR, sulfuric acid is diluted in the reactor as the nitrile hydrolysisreaction occurs. In either case, a separate acid dilution system is notrequired and associated installation and maintenance costs are avoided.The concentrated sulfuric acid introduced into the aqueous hydrolysismixture has a strength of between about 70% by weight and about 99% byweight, preferably between about 90% by weight and about 98% by weight.The aqueous hydrolysis mixture within the CSTR 10 has a strength ofbetween about 50% by weight and about 70% by weight, preferably betweenabout 60% by weight and about 70% by weight of sulfuric acid on anorganic-free basis. The aqueous hydrolysis mixture is continuouslypumped through an external heat exchanger 24 at a high circulation rateto remove heat of reaction. A pump 22 circulates the aqueous hydrolysismixture between CSTR 10 and an external heat exchanger 24, in whichexothermic heat of reaction is removed by transfer to a coolant. Theheat exchanger also removes the heat generated by dilution of sulfuricacid when concentrated sulfuric acid is fed directly to reactor 10.

68. The process of the present invention provides an improved method forpreparing HMBA. High productivity can be achieved using such a processbecause it can be operated continuously to provide greater throughputthan a conventional batch process. The process significantly reducescapital and maintenance costs associated with batch processes, forexample, by eliminating the need for separate sulfuric acid dilutionwhen concentrated sulfuric acid is introduced to a reactor without priordilution. The process also affords improved control of reactionconditions as compared to conventional batch hydrolysis systems. Suchimproved control of the hydrolysis reactions enables production of ahydrolyzate product of consistently high quality. The process ventemissions are significantly reduced as compared to conventional batchsystems because filling and emptying of tanks and operation atnon-steady state conditions is eliminated.

69. The following examples are presented to describe preferredembodiments and utilities of the present invention and are not meant tolimit the present invention unless otherwise stated in the claimsappended hereto.

EXAMPLE 1

70. Bench scale equipment as shown in FIG. 7 was used to demonstrate thecontinuous hydrolysis process.

71. Nitrile (2-hydroxy-4-methylthiobutanenitrile) and 65% aqueoussulfuric acid were continuously pumped at rates of 1.01 g/min and 1.167g/min, respectively, into a well-mixed recirculating reactor 46 having aliquid volume of 42.1 milliliters. The reaction temperature wascontrolled at 65° C. through cooling jackets provided on therecirculating reactor loop, which removed the heat released from thenitrile hydrolysis reaction. A pump 48 recirculated the aqueoushydrolysis mixture in the reactor loop. The residence time of thereactor 46 based on the total feed rate was 25.4 minutes. At the outletof the reactor, a sample was periodically removed during steady stateconditions. All sampling ports are designated with an S in FIG. 7. Thesample was analyzed by a gas chromatographic method to determine thehydrolyzate product composition leaving the reactor. The gaschromatography result showed that practically all nitrile feed washydrolyzed and converted to amide and approximately 15% of the formedamide was further hydrolyzed in this reactor to form HMBA, the finalhydrolysis product.

72. The amide rich hydrolyzate leaving the recirculating reactor 46 wasfed continuously into the second recirculating reactor 50 which issimilar to the first recirculating reactor 46 but has a liquid volume of119.3 milliliters. A water feed at 0.57 g/min was also introduced intothe second well-mixed reactor that provided a residence time of 52.6minutes. The temperature of this reactor loop was maintained at 102° C.via a heating fluid jacket provided on the recirculating reactor loop. Apump 52 recirculated the hydrolyzate in the reactor loop. An outletsample from the reactor 50 was obtained and analyzed by gaschromatography which revealed that approximately 94.5% of the feed amidewas hydrolyzed to HMBA.

73. The outlet from the second recirculating reactor 50 continuouslyentered the final finishing reactor that was constructed of a series offour coils 54 of Teflon tubing. The finishing reactor was placed insidea constant temperature oven 56 for preventing heat losses to theambient, thus maintaining a temperature of 102° C. throughout thereactor coils 54. This isothermal PFR having a total liquid volume of 91milliliters and a corresponding 43 minutes residence time was designedto assure completion of the amide hydrolysis. In this case, thehydrolysis of amide was completed at the outlet of the third coil 58.The hydrolyzate product taken from the outlet of the PFR was analyzedand contained 35% HMBA, with the remaining material being water andby-product ammonium bisulfate. The color of the hydrolyzate product was6-7 on the Gardner color scale.

EXAMPLES 2-9

74. The same continuous bench scale equipment as used in Example 1 wasalso used to determine the effect of residence time and reactiontemperature on conversion. The acid to nitrile feed ratio of eachexample was maintained at approximately a 1.0 molar stoichiometricratio. At the outlet of each recirculating reactor and the end of eachcoil of the PFR, a sample (indicated as RECIRC and S, respectively, inTables 1-8 below) was removed during steady state conditions and wasanalyzed by a gas chromatographic method to determine the hydrolysismixture composition leaving the reactor or coil. The hydrolysis mixturecomposition and the temperature and residence time in each reactor orcoil are shown in Tables 1-8 below. The remainder of the productincluded water and ammonium bisulfate. The results, based on variousfeed rates (1.01-2.33 grams/min. nitrile feed) and temperatures (60-65°C. for nitrile hydrolysis and 90-120° C. for amide hydrolysis)illustrate that increasing residence time and reaction temperaturesimproves the conversion of both hydrolysis reactions. However,increasing temperatures also resulted in an increase in product color.

EXAMPLE 2

75. Nitrile at 1.01 g/min was fed to the first recirculating reactor,along with 1.15 g/min 64.7% sulfuric acid, giving a 0.99 acid/nitrilemolar ratio. A water feed at 0.55 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 1 below. TABLE 1 RECIRC-I RECIRC-II S1 S2 S3 Temperature 64 103104 104 104 (° C.) Residence 25 53 11 11 11 Time (Min) HydrolysisMixture Composition (wt. %) Nitrile 0.11 trace trace trace trace HMBA8.3 34 33 35 33 Amide 38 2.7 0.70 0.12 0.03

EXAMPLE 3

76. Nitrile at 1.01 g/min was fed to the first recirculating reactor,along with 1.16 g/min 64.7% sulfuric acid, giving a 0.99 acid/nitrilemolar ratio. A water feed at 0.54 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 2 below. TABLE 2 RECIRC-I RECIRC-II S1 S2 S3 Temperature 62 98 102102 102 (° C.) Residence 25 53 11 11 11 Time (Min) Hydrolysis MixtureComposition (wt. %) Nitrile 0.22 0.05 0.05 trace 0.035 HMBA 7.8 33 38 3839 Amide 35 3.5 0.81 0.18 0.09

EXAMPLE 4

77. Nitrile at 1.43 g/min was fed to the first recirculating reactor,along with 1.65 g/min 64.7% sulfuric acid, giving a 0.99 acid/nitrilemolar ratio. A water feed at 0.76 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 3 below. TABLE 3 RECIRC-I RECIRC-II S1 S2 S3 Temperature 65 105105 105 105 (° C.) Residence 18 36 7.7 7.7 7.7 Time (Min) HydrolysisMixture Composition (wt. %) Nitrile 0.36 0.06 0.05 trace trace HMBA 6.734 36 37 38 Amide 39 3.1 0.79 0.28 trace

EXAMPLE 5

78. Nitrile at 1.45 g/min was fed to the first recirculating reactor,along with 1.69 g/min 64.7% sulfuric acid, giving a 1.0 acid/nitrilemolar ratio. A water feed at 0.78 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 4 below. TABLE 4 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 65 9090 90 90 90 (° C.) Residence 18 37 7.7 7.7 7.7 7.3 Time (Min) HydrolysisMixture Composition (wt. %) Nitrile 0.37 trace trace trace trace traceHMBA 6.1 32 36 37 36 37 Amide 40 5.8 2.1 0.99 0.60 0.40

EXAMPLE 6

79. Nitrile at 2.0 g/min was fed to the first recirculating reactor,along with 2.33 g/min 65% sulfuric acid, giving a 1.01 acid/nitrilemolar ratio. A water feed at 1.09 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 5 below. TABLE 5 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 65 105105 105 105 105 (° C.) Residence 13 26 5.4 5.4 5.4 5.2 Time (Min)Hydrolysis Mixture Composition (wt. %) Nitrile 0.45 0.09 trace 0.04trace 0.06 HMBA 5.3 34 36 36 36 37 Amide 40 3.3 0.84 0.28 0.12 0.07

EXAMPLE 7

80. Nitrile at 1.42 g/min was fed to the first recirculating reactor,along with 1.65 g/min 65% sulfuric acid, giving a 1.01 acid/nitrilemolar ratio. A water feed at 0.795 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 6 below. TABLE 6 RECIRC-I RECIRC-II S1 S2 S3 Temperature 60 120120 120 120 (° C.) Residence 18 36 7.5 7.5 7.5 Time (Min) HydrolysisMixture Composition (wt. %) Nitrile 0.23 0.05 trace trace trace HMBA 5.236 35 36 36 Amide 39 1.5 trace trace trace

EXAMPLE 8

81. Nitrile at 1.43 g/min was fed to the first recirculating reactor,along with 1.66 g/min 65% sulfuric acid, giving a 1.0 acid/nitrile molarratio. A water feed at 0.78 g/min was also introduced into the secondrecirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 7 below. TABLE 7 RECIRC-I RECIRC-II S1 S2 S4 Temperature 65 100100 100 100 (° C.) Residence 18 36 7.6 7.6 7.3 Time (Min) HydrolysisMixture Composition (wt. %) Nitrile 0.26 0.10 NA NA trace HMBA 7.0 34 NANA 37 Amide 36 3.7 NA NA 0.05

EXAMPLE 9

82. Nitrile at 1.04 g/min was fed to the first recirculating reactor,along with 1.15 g/min 65% sulfuric acid, giving a 0.96 acid/nitrilemolar ratio. A water feed at 0.57 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 8 below. TABLE 8 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 65 100100 100 100 100 (° C.) Residence 25 52 11 11 11 11 Time (Min) HydrolysisMixture Composition (wt. %) Nitrile 0.36 0.08 0.05 0.05 0.04 trace HMBA8.7 35 37 36 35 34 Amide 39 3.6 0.89 0.37 0.17 0.05

EXAMPLES 10-20

83. The effect of acid/nitrile feed molar ratio on the reactionconversion, as well as the coupling effect of this ratio with reactiontemperature was determined. In these examples, the nitrile feed rate wasessentially constant and the water feed rate was adjusted for various65% sulfuric acid feeds to assure the same water content of the finalhydrolyzate from each run. At the outlet of each reactor and the end ofeach coil of the PFR, a sample was removed during steady stateconditions and was analyzed by a gas chromatographic method to determinethe hydrolyzate product composition leaving the reactor or coil. Thehydrolysis mixture composition and the temperature and residence time ineach reactor or coil are shown below. The remainder of the hydrolyzateincluded water and ammonium bisulfate. Based on the range of thevariables that were analyzed, i.e., acid/nitrile molar ratio from 0.6 to1.2 and amide hydrolysis temperature from 90-120° C., an optimum rangeof conditions were derived as shown in FIG. 8 for the fixed residence(or nitrile feed rate) tested. Within the range of 90-101° C. and1.0-1.2 acid/nitrile ratio, any combination of temperature andacid/nitrile molar ratio will result in a satisfactory productcontaining up to 0.05% by weight amide, up to 0.05% by weight nitrileand having a color of between 5 and 7 on a Gardner scale.

EXAMPLE 10

84. Nitrile at 1.02 g/min was fed to the first recirculating reactor,along with 1.03 g/min 64.75% sulfuric acid, giving a 0.88 acid/nitrilemolar ratio. A water feed at 0.53 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 9 below. TABLE 9 RECIRC-I RECIRC-II S1 S2 S3/S4 Temperature 65 105105 105 105 (° C.) Residence 27 53 11 11 11 Time (Min) HydrolysisMixture Composition (wt. %) Nitrile 0.80 0.31 0.35 0.40 NA HMBA 8.3 3541 49 NA Amide 41 3.7 1.7 0.96 NA

EXAMPLE 11

85. Nitrile at 0.99 g/min was fed to the first recirculating reactor,along with 0.70 g/min 65% sulfuric acid, giving a 0.62 acid/nitrilemolar ratio. A water feed at 0.94 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 10 below. TABLE 10 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 6590 90 90 90 90 (° C.) Residence 32 53 11 11 11 11 Time (Min) HydrolysisMixture Composition (wt. %) Nitrile 5.2 2.2 2.4 2.5 2.5 2.5 HMBA 7.9 2527 29 30 30 Amide 45 9.5 7.4 5.9 5.2 4.6

EXAMPLE 12

86. Nitrile at 1.01 g/min was fed to the first recirculating reactor,along with 1.37 g/min 64.75% sulfuric acid, giving a 1.19 acid/nitrilemolar ratio. A water feed at 0.53 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 11 below. TABLE II RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 6590 90 90 90 90 (° C.) Residence 23 50 10 10 10 10 Time (Min) HydrolysisMixture Composition (wt. %) Nitrile trace trace trace trace trace traceHMBA 7.7 31 35 34 35 35 Amide 33 2.8 0.72 0.17 0.03 trace

EXAMPLE 13

87. Nitrile at 1.01 g/min was fed to the first recirculating reactor,along with 0.70 g/min 65% sulfuric acid, giving a 0.60 acid/nitrilemolar ratio. A water feed at 0.90 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 12 below. TABLE 12 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 65120 120 120 120 120 (° C.) Residence 32 52 11 11 11 10 Time (Min)Hydrolysis Mixture Composition (wt. %) Nitrile 6.0 2.6 2.7 2.2 2.7 2.3HMBA 7.7 27 32 29 34 31 Amide 44 5.4 4.0 1.4 1.8 1.4

EXAMPLE 14

88. Nitrile at 1.0 g/min was fed to the first recirculating reactor,along with 1.37 g/min 64.75% sulfuric acid, giving a 1.19 acid/nitrilemolar ratio. A water feed at 0.513 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 13 below. TABLE 13 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 65120 120 120 120 120 (° C.) Residence 23 50 10 10 10 10 Time (Min)Hydrolysis Mixture Composition (wt. %) Nitrile trace trace trace tracetrace trace HMBA 8.5 34 34 35 35 34 Amide 31 0.44 trace trace tracetrace

EXAMPLE 15

89. Nitrile at 1.0 g/min was fed to the first recirculating reactor,along with 1.05 g/min 64.75% sulfuric acid, giving a 0.91 acid/nitrilemolar ratio. A water feed at 0.67 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 14 below. TABLE 14 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 65105 105 105 105 105 (° C.) Residence 27 53 11 11 11 11 Time (Mm)Hydrolysis Mixture Composition (wt. %) Nitrile 0.39 0.11 0.11 0.11 0.100.09 HMBA 8.9 34 34 37 38 38 Amide 37 4.0 1.5 0.71 0.32 0.20

EXAMPLE 16

90. Nitrile at 1.02 g/min was fed to the first recirculating reactor,along with 0.71 g/min 64.75% sulfuric acid, giving a 0.6 acid/nitrilemolar ratio. A water feed at 0.93 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 15 below. TABLE 15 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 65120 120 120 120 120 (° C.) Residence 32 52 11 11 11 10 Time (Min)Hydrolysis Mixture Composition (wt. %) Nitrile 5.7 2.5 2.7 2.6 2.6 2.5HMBA 8.5 29 33 34 35 35 Amide 45 6.2 4.3 2.6 2.0 1.5

EXAMPLE 17

91. Nitrile at 1.02 g/min was fed to the first recirculating reactor,along with 0.69 g/min 65% sulfuric acid, giving a 0.59 acid/nitrilemolar ratio. A water feed at 0.90 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 16 below. TABLE 16 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 6590 90 90 90 90 (° C.) Residence 32 53 11 11 11 10 Time (Mm) HydrolysisMixture Composition (wt. %) Nitrile 6.0 3.4 3.2 3.2 3.3 3.3 HMBA 8.2 2527 28 29 30 Amide 44 12 8.0 7.5 6.6 5.7

EXAMPLE 18

92. Nitrile at 1.02 g/min was fed to the first recirculating reactor,along with 1.38 g/min 65% sulfuric acid, giving a 1.18 acid/nitrilemolar ratio. A water feed at 0.54 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 17 below. TABLE 17 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 6590 90 90 90 90 (° C.) Residence 23 50 10 10 10 10 Time (Mm) HydrolysisMixture Composition (wt. %) Nitrile trace trace trace trace trace traceHMBA 7.2 31 35 35 35 36 Amide 34 2.9 0.75 0.24 trace trace

EXAMPLE 19

93. Nitrile at 1.03 g/min was fed to the first recirculating reactor,along with 1.39 g/min 65% sulfuric acid, giving a 1.17 acid/nitrilemolar ratio. A water feed at 0.52 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 18 below. TABLE 18 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 65120 120 120 120 120 (° C.) Residence 23 50 10 10 10 10 Time (Min)Hydrolysis Mixture Composition (wt. %) Nitrile trace trace trace tracetrace trace HMBA 7.1 33 33 34 34 48 Amide 35 0.39 trace trace tracetrace

EXAMPLE 20

94. Nitrile at 1.02 g/min was fed to the first recirculating reactor,along with 1.05 g/min 65% sulfuric acid, giving a 0.90 acid/nitrilemolar ratio. A water feed at 0.63 g/min was also introduced into thesecond recirculating reactor. The hydrolysis mixture composition and thetemperature and residence time in each reactor or coil are shown inTable 19 below. TABLE 19 RECIRC-I RECIRC-II S1 S2 S3 S4 Temperature 65105 105 105 105 105 (° C.) Residence 27 53 11 11 11 11 Time (Min)Hydrolysis Mixture Composition (wt. %) Nitrile 0.38 0.06 0.09 0.09 0.090.09 HMBA 8.9 28 39 31 41 38 Amide 40 2.5 0.97 0.31 0.18 0.10

EXAMPLE 21

95. The bench scale equipment used in the preceding examples wasmodified by replacing the second recirculating reactor 30 with a smallmixing loop 60 with negligible volume used for mixing the water feed andthe hydrolysis mixture leaving the first loop reactor 46. The finishingreaction stream was recirculated through the mixing loop by a pump 62.The mixer loop 60 was heated in a hot water bath 64 in order to heat thefinishing reaction stream before it entered the PFR in which the amidehydrolysis reaction occurred. The modified bench scale equipment isshown in FIG. 9.

96. Nitrile at 0.73 g/min was fed to the first recirculating reactor,along with 0.83 g/min 65% sulfuric acid, giving a 0.99 acid/nitrilemolar ratio. The temperature in the reactor loop was 60° C. and theresidence time was estimated as 36.8 minutes. Analysis of the reactoroutlet sample revealed that nitrile was essentially hydrolyzed to amidewith less than 0.1% unreacted nitrile remaining in the outlet stream.The temperature of the hydrolysis mixture at the outlet of the mixerloop was 75° C. and the residence time in the mixer loop was 1.5minutes. The PFR coils were maintained at 100 to 101° C. The residencetime in each of the first three coils was 16 minutes and that in thelast coil was 15.2 minutes. Amide hydrolysis was completed in the lastPFR coil.

EXAMPLE 22

97. The equipment used in the continuous hydrolysis process as shown inFIG. 3 consists of two CSTRs and one packed column type PFR. The firstCSTR was devoted for the nitrile hydrolysis while the second CSTR andPFR were for the amide hydrolysis. The PFR is an 8 inch diameter teflonlined carbon steel pipe packed with Koch SMVP Teflon packing. The PFRwas manufactured by Koch Engineering. The threshold velocity for theSMVP packing is 0.95 mm/sec.

98. 105 lbs/hr of nitrile and 121 lbs/hr of 65% sulfuric acid werecontinuously fed to the first 20 gallon CSTR in which 13 gallons ofliquid were maintained by a level controller controlling the reactoroutlet flow. The reactor was maintained at 65° C. by an external coolerin a product recirculating loop. The residence time based on the totalfeed rate and the reactor liquid volume was 38 minutes. The outlethydrolyzate sample, based on a gas chromatographic analysis, was foundto contain less than 0.1% nitrile, 34.9% amide, and 11.2% HMBA. Theoutlet stream was introduced to the second 30 gallon CSTR having aliquid volume of 27.7 gallons. An 80° C. hot water stream was also fedto the second CSTR at a rate of 60.5 lbs/hr. The reactor temperature was105° C. and the residence time was 91 minutes. The hydrolyzate from thereactor contained 1.9% amide, indicating that more than 90% of incomingamide was converted to HMBA in this vessel. The second CSTR outletstream then entered the packed column reactor containing structurepacking and having a total liquid volume of 25 gallons. From the varioussamples obtained along the length of the column reactor, the amidehydrolysis reaction was found to have approached completion at 70% ofthe length of the reactor. The temperature profile of the adiabaticcolumn reactor ranged from 100 to 102° C. and the residence time in thePFR was 52.9 minutes. The final product contained less than 0.1%nitrile, less than 0.1% amide, and 48% HMBA. The major by-product of thehydrolysis process, ammonium bisulfate, can be separated from theproduct by conventional means.

EXAMPLE 23

99. The equipment as used in Example 22 was used for the followinghydrolysis process except that the second CSTR was bypassed such thatthe packed column reactor was the sole reactor for the amide hydrolysisreaction.

100. The feed rates to the first CSTR were as described in Example 22.However, the temperature in the first CSTR was 60° C. Analysis of theoutlet sample revealed that the intermediate hydrolysis mixturecontained 0.2% nitrile, 39.4% amide, and 9.5% HMBA. The lower CSTRoperating temperature resulted in a slightly higher nitrileconcentration but a lower HMBA concentration. The intermediatehydrolysis mixture was mixed with a hot water stream (60.5 lbs/hr) in anin-line static mixer. The finishing reaction stream entered the PFR thatmaintained a steady state temperature profile from 80° C. at the inlet,reaching a peak temperature of 105° C. at the middle and dropping to102° C. at the outlet of the packed column. Although the column wallswere heat traced and insulated, some heat losses were encountered. Theresidence time in the column reactor was 52.9 minutes. The finalhydrolyzate at the outlet of the column reactor contained less than 0.1%nitrile, 0.1% amide and 40.8% HMBA, the balance being by-productammonium bisulfate and water.

EXAMPLE 24

101. The equipment as utilized in Example 23 was used in the followinghydrolysis except that concentrated sulfuric acid was fed directly tothe first CSTR (FIG. 5) without pre-dilution to 65% sulfuric acid withwater. A water stream was also fed to the reactor. Thus, both the heatof dilution of the acid and the heat of hydrolysis were removed via theexternal circulating cooler. The second CSTR was by-passed.

102. Nitrile (72 lbs/hr), 96% sulfuric acid (56.2 lbs/hr) and dilutionwater (26.7 lbs/hr) were simultaneously fed to the first CSTR wherenitrile hydrolysis was occurring. The operating liquid volume was 10gallons, which provided 42.5 minutes of residence time based on thetotal sum of the three feed rates. The reaction temperature wascontrolled at 55° C. The gas chromatographic analysis of the aqueoushydrolysis mixture showed that it contained 0.5% nitrile, 40.6% amide,and 5.7% HMBA. The aqueous hydrolysis mixture was mixed with 41.3 lbs/hrhot water in the in-line static mixer. The finishing reaction streamentered the packed column reactor in the same fashion as described inExample 23, except that the adiabatic reaction temperatures in thecolumn reactor were slightly higher, probably due to additional heatrelease from the higher unreacted nitrile content leaving the CSTR thatwas operated at a lower temperature. From the samples withdrawn from thecolumn reactor, the amide hydrolysis was determined to have beencompleted at 70% of the column height from the bottom inlet.

EXAMPLES 25-38

103. Hydrolysis mixture samples were taken at the outlet of each CSTR,the PFR inlet (S1), the PFR outlet (S6), and at four sampling portsalong the length of the PFR (S2 through S5) as shown in FIG. 3. Thesamples were removed during steady state conditions and were analyzed bya gas chromatographic method to determine the hydrolysis mixturecomposition when leaving the CSTRs and when flowing through the PFR. Thehydrolysis mixture composition and the temperature and residence time ineach CSTR and within each section of the PFR are shown below. Theremainder of the hydrolysis mixture included water and ammoniumbisulfate. Examples that do not indicate CSTR-II data involved equipmentwherein the second CSTR was bypassed such that a diluted aqueoushydrolysis mixture flowed from the in-line mixer directly to the PFR.

104. The data demonstrate that conversion is affected by temperature,acid/nitrile ratio and the degree of axial back mixing in the plug flowreactor. Back mixing is in turn a function of the velocity at which thereacting mixture flows through the reactor. In the instances in whichback mixing affected conversion, the reactor was operated at less thanits threshold velocity of 1.0 mm/sec., resulting in a lower averagedriving force, i.e., amide concentration integrated along the length ofthe reactor, for this non-zero order reaction. In some instances, it waspossible to compensate for operation below threshold velocity usingrelatively higher temperature and/or acid/nitrile ratio. Furtherdiscussion of the relationship of velocity to axial back mixing and theresultant effect on conversion is set forth at the end of Example 38.

EXAMPLE 25

105. Nitrile at 105.00 lbs/hr was fed to the first CSTR, along with120.95 lbs/hr 65% sulfuric acid, giving a 1.03 acid/nitrile molar ratio.A water feed at 60.50 lbs/hr was also introduced into the second CSTR.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 20 below. TABLE 20 CSTR- CSTR-I IIS1 S2 S3 S4 S5 S6 Volume 13 27.7 17.5 (total PFR) (gal) Temper- 65 104101 103 102 103 103 102 ature (° C.) Hydrolysis Mixture Composition (wt.%) Nitrile 0.02 0.02 0.01 0.01 0.01 trace trace trace HMBA 12 41 34 3939 39 41 40 Amide 34 2.0 0.5 0.27 0.04 0.02 0.02 0.02

EXAMPLE 26

106. Nitrile at 105.00 lbs/hr was fed to the first CSTR, along with120.95 lbs/hr 65% sulfuric acid, giving a 1.00 acid/nitrile molar ratio.A water feed at 60.50 lbs/hr was also introduced into the second CSTR.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 21 below. TABLE 21 CSTR- CSTR- I IIS1 S2 S3 S4 S5 S6 Volume 13 27.7 17.5 (total PFR) (gal) Tempera- 65 105101 102 102 102 102 101 ture (° C.) Hydrolysis Mixture Composition (wt.%) Nitrile 0.01 trace trace trace trace trace trace 0.01 HMBA 11 36 3729 26 30 31 47 Amide 35 1.9 1.1 0.21 0.05 0.05 0.05 0.05

EXAMPLE 27

107. Nitrile at 105.00 lbs/hr was fed to the first CSTR, along with145.14 lbs/hr 65% sulfuric acid, giving a 1.21 acid/nitrile molar ratio.A water feed at 57.4 lbs/hr was also introduced into the second CSTR.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 22 below. TABLE 22 CSTR- CSTR- I IIS1 S2 S3 S4 S5 S6 Volume 13 27.7 17.5 (total PFR) (gal) Tempera- 65 9390 93 92 92 92 93 ture (° C.) Hydrolysis Mixture Composition (wt. %)Nitrile trace trace trace trace trace trace trace trace HMBA 11 33 34 3536 36 38 33 Amide 31 1.8 1.3 0.14 0.02 0.01 0.01 0.01

EXAMPLE 28

108. Nitrile at 72.00 lbs/hr was fed to the first CSTR, along with 82.94lbs/hr 65% sulfuric acid, giving a 1.0 acid/nitrile molar ratio. A waterfeed at 41.30 lbs/hr was also introduced into the in-line mixer. Thehydrolysis mixture composition for each sample and the temperature foreach location are shown in Table 23 below. TABLE 23 CSTR-I S1 S2 S3 S4S5 S6 Volume (gal) 10 19.9 (total PFR) Temperature 65 80 96 97 97 97 97(° C.) Hydrolysis Mixture Composition (wt. %) Nitrile 0.13 0.03 0.020.01 0.01 0.01 0.02 HMBA 14 22 28 24 27 25 44 Amide 40 19 3.3 1.6 1.71.4 1.2

EXAMPLE 29

109. Nitrile at 72.00 lbs/hr was fed to the first CSTR, along with 82.94lbs/hr 65% sulfuric acid, giving a 1.0 acid/nitrile molar ratio. A waterfeed at 41.30 lbs/hr was also introduced into the in-line mixer. Thehydrolysis mixture composition for each sample and the temperature foreach location are shown in Table 24 below. TABLE 24 CSTR-I S1 S2 S3 S4S5 S6 Volume (gal) 10 19.9 (total PFR) Temperature 60 81 103 103 101 101100 (° C.) Hydrolysis Mixture Composition (wt. %) Nitrile 0.19 0.01trace trace trace trace trace HMBA 8.8 20 38 37 39 47 54 Amide 38 22 1.40.59 0.34 0.33 0.06

EXAMPLE 30

110. Nitrile at 150.00 lbs/hr was fed to the first CSTR, along with172.79 lbs/hr 65% sulfuric acid, giving a 1.04 acid/nitrile molar ratio.A water feed at 86.40 lbs/hr was also introduced into the second CSTR.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 25 below. TABLE 25 CSTR- CSTR- I IIS1 S2 S3 S4 S5 S6 Volume 10 27.7 25.0 (total PFR) (gal) Tempera- 65 104100 102 102 102 101 ture (° C.) Hydrolysis Mixture Composition (wt. %)Nitrile 0.26 0.03 0.01 trace trace trace trace trace HMBA 8.1 35 37 3937 37 39 38 Amide 38 3.0 1.5 0.21 0.03 0.02 0.02 0.02

EXAMPLE 31

111. Nitrile at 150.00 lbs/hr was fed to the first CSTR, along with172.79 lbs/hr 65% sulfuric acid, giving a 1.02 acid/nitrile molar ratio.A water feed at 86.40 lbs/hr was also introduced into the second CSTR.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 26 below. TABLE 26 CSTR- CSTR- I IIS1 S2 S3 S4 S5 S6 Volume 10 27.7 25.0 (total PFR) (gal) Tempera- 65 102100 103 103 104 103 102 ture (° C.) Hydrolysis Mixture Composition (wt.%) Nitrile 0.13 0.01 0.03 0.03 0.02 0.02 0.02 0.01 HMBA 7.0 36 37 39 3940 41 40 Amide 40 3.2 2.4 0.84 0.23 0.18 0.16 0.13

EXAMPLE 32

112. Nitrile at 105.00 lbs/hr was fed to the first CSTR, along with120.95 lbs/hr 65% sulfuric acid, giving a 1.02 acid/nitrile molar ratio.A water feed at 60.50 lbs/hr was also introduced into the second CSTR.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 27 below. TABLE 27 CSTR- CSTR- I IIS1 S2 S3 S4 S5 S6 Volume 10 27.7 17.5 (total PFR) (gal) Tempera- 63 105101 103 103 103 103 101 ture (° C.) Hydrolysis Mixture Composition (wt.%) Nitrile 0.36 0.03 0.01 0.01 0.01 trace trace trace HMBA 9.1 38 38 4040 41 39 40 Amide 39 2.7 1.8 0.32 0.08 0.05 0.05 0.03

EXAMPLE 33

113. Nitrile at 72.00 lbs/hr was fed to the first CSTR, along with 82.94lbs/hr 65% sulfuric acid, giving a 1.0 acid/nitrile molar ratio. A waterfeed at 41.30 lbs/hr was also introduced into the in-line mixer. Thehydrolysis mixture composition for each sample and the temperature foreach location are shown in Table 28 below. TABLE 28 CSTR-I S1 S2 S3 S4S5 S6 Volume (gal) 10 19.9 (total PFR) Temperature 60 84 103 105 105 105103 (° C.) Hydrolysis Mixture Composition (wt. %) Nitrile 0.41 0.20 0.010.01 0.01 0.01 0.01 HMBA 9.5 23 26 23 23 23 47 Amide 43 17 2.3 0.63 0.620.61 0.72

EXAMPLE 34

114. Nitrile at 90.60 lbs/hr was fed to the first CSTR, along with103.80 lbs/hr 65% sulfuric acid, giving a 1.03 acid/nitrile molar ratio.A water feed at 51.90 lbs/hr was also introduced into the in-line mixer.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 29 below. TABLE 29 CSTR-I S1 S2 S3S4 S5 S6 Volume (gal) 12.6 25.0 (total PFR) Temperature 59 82 104 105105 104 104 (° C.) Hydrolysis Mixture Composition (wt. %) Nitrile 0.390.03 0.01 0.01 0.01 0.01 0.01 HMBA 9.1 23 33 30 31 31 38 Amide 41 18 2.00.92 0.79 0.71 0.76

EXAMPLE 35

115. Nitrile at 90.60 lbs/hr was fed to the first CSTR, along with124.80 lbs/hr 65% sulfuric acid, giving a 1.20 acid/nitrile molar ratio.A water feed at 49.40 lbs/hr was also introduced into the in-line mixer.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 30 below. TABLE 30 CSTR-I S1 S2 S3S4 S5 S6 Volume (gal) 12.6 25.0 (total PFR) Temperature 45 86 107 107107 107 105 (° C.) Hydrolysis Mixture Composition (wt. %) Nitrile 0.070.01 trace trace trace trace trace HMBA 8.1 23 35 33 39 37 37 Amide 3612 1.0 trace trace trace 0.01

EXAMPLE 36

116. Nitrile at 90.60 lbs/hr was fed to the first CSTR, along with124.80 lbs/hr 65% sulfuric acid, giving a 1.20 acid/nitrile molar ratio.A water feed at 49.40 lbs/hr was also introduced into the in-line mixer.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 31 below. TABLE 31 CSTR-I S1 S2 S3S4 S5 S6 Volume (gal) 12.6 25.0 (total PFR) Temperature 58 82 105 103103 103 101 (° C.) Hydrolysis Mixture Composition (wt. %) Nitrile tracetrace trace trace trace trace trace HMBA 8.3 20 38 37 38 38 39 Amide 3118 0.96 trace trace trace trace

EXAMPLE 37

117. Nitrile at 90.60 lbs/hr was fed to the first CSTR, along with103.80 lbs/hr 65% sulfuric acid, giving a 1.0 acid/nitrile molar ratio.A water feed at 51.90 lbs/hr was also introduced into the in-line mixer.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 32 below. TABLE 32 CSTR-I S1 S2 S3S4 S5 S6 Volume 12.6 25.0 (total PFR) (gal) Temperature 59 88 109 109109 109 107 (° C.) Hydrolysis Mixture Composition (wt. %) Nitrile 0.090.01 trace 0.01 trace trace 0.01 HMBA 8.8 26 38 69 38 40 41 Amide 37 130.30 0.02 0.01 0.01 trace

EXAMPLE 38

118. Nitrile at 105.00 lbs/hr was fed to the first CSTR, along with120.95 lbs/hr 65% sulfuric acid, giving a 1.0 acid/nitrile molar ratio.A water feed at 60.50 lbs/hr was also introduced into the in-line mixer.The hydrolysis mixture composition for each sample and the temperaturefor each location are shown in Table 33 below. TABLE 33 CSTR-I S1 S2 S3S4 S5 S6 Volume 14.6 25.0 (total PFR) (gal) Temperature 60 80 103 104105 104 102 (° C.) Hydrolysis Mixture Composition (wt. %) Nitrile 0.220.02 0.01 0.01 0.01 0.01 0.01 HMBA 9.5 24 39 45 45 42 41 Amide 39 21 2.50.19 0.20 0.28 0.11

119. As noted above, certain of the conversions obtained in a packedcolumn PFR were not sufficient to meet target residual amideconcentrations in the product hydrolyzate. These lower conversions wereattributable to lower reaction temperature or acid/nitrile ratio,excessive axial back mixing, or some combination of these factors. Basedon studies conducted on reactors operated at velocity adequate toprovide a Peclet number greater than about 50, it was determined thatresidual amide concentration in the hydrolyzate could be consistentlyreduced to less than about 0.03% at reaction temperatures andacid/nitrile ratios in the preferred ranges discussed above for steadystate operations. But where the Peclet number was significantly below50, lower conversions were generally found unless temperature and/oracid/nitrile ratio were increased to compensate.

120. To investigate the effect of velocity on back-mixing in a packedcolumn PFR, residence time distribution tests were conducted at varyingvelocities using a pulse of salt as a tracer injected at the bottom of acolumn in which tap water was caused to flow upwardly. At the top outletof the reactor a conductivity probe was inserted for measuring theconductivity of the outlet flow, from which tracer response data, interms of salt (NaCl) concentration vs. time, were obtained. Followingconventional methods, calculations based on the response data were madeto determine the mean residence time (the first moment of distribution),the variance (the second moment of distribution), the Peclet number, andthe equivalent number of stirred tanks in series. For the reactortested, the flow rate (gpm), mean residence time (θ), dimensionlessvariance (σ²), Peclet number (Pe), and number of equivalent stirred tankreactors (j), are set forth in Table 34. TABLE 34 gpm θ σ² Pe j 0.9529.5 0.0393 50.9 25.5 0.47 66.3 0.0681 29.4 14.7 0.90 25.1 0.0749 26.713.4*

121. These data demonstrate a critical velocity threshold in the rangeof 0.5 gpm for the packed column that was used in these tests.

122. Based on kinetic calculations on the amide hydrolysis reaction, therelationship between the number of equivalent stirred tank reactors andthe residual amide concentration was calculated. Computations were alsomade of the correlation between the number of equivalent stirred tankreactors and: (a) the ratio of (requisite reactor length for a givendegree of conversion) to (requisite length for the same degree ofconversion under perfect plug flow conditions) (L/L_(p)); and (b) theratio between (residual amide concentration for a given length ofreactor) vs. (residual amide concentration for the same length reactorunder perfect plug flow conditions) (C/C_(p)). These calculations areset forth in Table 35. TABLE 35 j L/L_(p) C/C_(p) C (% amide out) 151.236 2.66 0.0581% 20 1.177 2.25 0.0491 25 1.141 2.00 0.0436 30 1.1781.83 0.0399 40 1.088 1.62 0.0353 ∞ 1.000 1.00 0.0218

EXAMPLES 39-55

123. Hydrolysis mixture samples were removed during steady stateconditions and were analyzed by a gas chromatographic method todetermine the hydrolysis mixture composition when leaving the CSTR,entering the PFR and leaving the PFR as shown in FIG. 2 for Examples39-48 and as shown in FIG. 1 for Examples 49-55.

124. The data demonstrate that conversion of HMBN to amide is improvedby an additional residence time of about 3 minutes within the zonebetween the outlet of the CSTR and the point of dilution before transferof the stream to the PFR. Examples 44 and 51 are directly comparable, asare Examples 45, 46 and 53 because they were run at the same flow rateand exhibited the same peak temperature in the PFR. Examples 39-48demonstrate the hydrolyzate color improvement as flow reactor peaktemperature was reduced from 105 or 106° C. to 101 or 102° C.

EXAMPLE 39

125. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture contained 0.03% nitrile and 39% amide byweight. The residence time of the aqueous hydrolysis mixture in thecirculation zone upstream of the forward flow port and in the forwardflow reaction zone between the forward flow port and the point ofdilution was 4 seconds and 3 minutes, respectively. After dilution, thefinishing reaction stream was fed to the PFR which operatedadiabatically at 81-106° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.01% amide and less than0.01% nitrile. The finished aqueous hydrolyzate product color was 8.5 ona Gardner scale.

EXAMPLE 40

126. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture contained 0.01% nitrile and 38% amide byweight. The residence time of the aqueous hydrolysis mixture in thecirculation zone upstream of the forward flow port and in the forwardflow reaction zone between the forward flow port and the point ofdilution was 4 seconds and 3 minutes, respectively. After dilution, thefinishing reaction stream was fed to the PFR which operatedadiabatically at 81-105° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.01% amide and less than0.01% nitrile. The finished aqueous hydrolyzate product color was 8.5 ona Gardner scale.

EXAMPLE 41

127. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture contained 0.08% nitrile and 38% amide byweight. The residence time of the aqueous hydrolysis mixture in thecirculation zone upstream of the forward flow port and in the forwardflow reaction zone between the forward flow port and the point ofdilution was 4 seconds and 3 minutes, respectively. After dilution, thefinishing reaction stream was fed to the PFR which operatedadiabatically at 80-105° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained less than 0.01% amide andnitrile. The finished aqueous hydrolyzate product color was 8.5 on aGardner scale.

EXAMPLE 42

128. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture contained 0.08% nitrile and 38% amide byweight. The residence time of the aqueous hydrolysis mixture in thecirculation zone upstream of the forward flow port and in the forwardflow reaction zone between the forward flow port and the point ofdilution was 4 seconds and 3 minutes, respectively. After dilution, thefinishing reaction stream was fed to the PFR which operatedadiabatically at 80-105° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained less than 0.01% amide andnitrile. The finished aqueous hydrolyzate product color was 7.5 on aGardner scale.

EXAMPLE 43

129. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture was not analyzed. The residence time ofthe aqueous hydrolysis mixture in the circulation zone upstream of theforward flow port and in the forward flow reaction zone between theforward flow port and the point of dilution was 4 seconds and 3 minutes,respectively. After dilution, the finishing reaction stream was fed tothe PFR which operated adiabatically at 79-104° C. and a residence timeof 70 minutes. The finished aqueous hydrolyzate product contained 0.01%amide and less than 0.01% nitrile. The finished aqueous hydrolyzateproduct color was 8.5 on a Gardner scale.

EXAMPLE 44

130. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture was not analyzed. The residence time ofthe aqueous hydrolysis mixture in the circulation zone upstream of theforward flow port and in the forward flow reaction zone between theforward flow port and the point of dilution was 4 seconds and 3 minutes,respectively. After dilution, the finishing reaction stream was fed tothe PFR which operated adiabatically at 79-104° C. and a residence timeof 70 minutes. The finished aqueous hydrolyzate product contained lessthan 0.01% amide and nitrile. The finished aqueous hydrolyzate productcolor was 8.5 on a Gardner scale.

EXAMPLE 45

131. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture contained 0.04% nitrile and 40% amide byweight. The residence time of the aqueous hydrolysis mixture in thecirculation zone upstream of the forward flow port and in the forwardflow reaction zone between the forward flow port and the point ofdilution was 4 seconds and 3 minutes, respectively. After dilution, thefinishing reaction stream was fed to the PFR which operatedadiabatically at 77-102° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.01% amide and less than0.01% nitrile. The finished aqueous hydrolyzate product color was 8.5 ona Gardner scale.

EXAMPLE 46

132. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture contained 0.03% nitrile and 40% amide byweight. The residence time of the aqueous hydrolysis mixture in thecirculation zone upstream of the forward flow port and in the forwardflow reaction zone between the forward flow port and the point ofdilution was 4 seconds and 3 minutes, respectively. After dilution, thefinishing reaction stream was fed to the PFR which operatedadiabatically at 77-102° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.01% amide and less than0.01% nitrile. The finished aqueous hydrolyzate product color was 7.5 ona Gardner scale.

EXAMPLE 47

133. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture contained 0.06% nitrile and 37% amide byweight. The residence time of the aqueous hydrolysis mixture in thecirculation zone upstream of the forward flow port and in the forwardflow reaction zone between the forward flow port and the point ofdilution was 4 seconds and 3 minutes, respectively. After dilution, thefinishing reaction stream was fed to the PFR which operatedadiabatically at 75-101° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.01% amide and less than0.01% nitrile. The finished aqueous hydrolyzate product color was 6.5 ona Gardner scale.

EXAMPLE 48

134. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 2 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at60° C., the hydrolysis mixture contained 41% amide by weight and lessthan 0.09% nitrile. The residence time of the aqueous hydrolysis mixturein the circulation zone upstream of the forward flow port and in theforward flow reaction zone between the forward flow port and the pointof dilution was 4 seconds and 3 minutes, respectively. After dilution,the finishing reaction stream was fed to the PFR which operatedadiabatically at 75-101° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.01% amide and less than0.01% nitrile. The finished aqueous hydrolyzate product color was 6.5 ona Gardner scale.

EXAMPLE 49

135. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 1 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 46 minutes in the CSTR at62° C., the hydrolysis mixture contained 43% amide and 0.02% nitrile.The residence time of the aqueous hydrolysis mixture in the circulationzone upstream of the forward flow port and in the forward flow reactionzone between the forward flow port and the point of dilution was lessthan one second and less than one minute, respectively. After dilution,the finishing reaction stream was fed to the PFR which operatedadiabatically at 80-104° C. and a residence time of 88 minutes. Thefinished aqueous hydrolyzate product contained 0.02% nitrile and lessthan 0.01% amide. The finished aqueous hydrolyzate product color was 8on a Gardner scale.

EXAMPLE 50

136. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 1 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 49 minutes in the CSTR at62° C., the hydrolysis mixture contained 40% amide and 0.04% nitrile.The residence time of the aqueous hydrolysis mixture in the circulationzone upstream of the forward flow port and in the forward flow reactionzone between the forward flow port and the point of dilution was lessthan one second and less than one minute, respectively. After dilution,the finishing reaction stream was fed to the PFR which operatedadiabatically at 82-104° C. and a residence time of 92 minutes. Thefinished aqueous hydrolyzate product contained 0.03% nitrile and 0.01%amide. The finished aqueous hydrolyzate product color was 8.5 on aGardner scale.

EXAMPLE 51

137. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 1 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at62° C., the hydrolysis mixture contained 35% amide and 0.03% nitrile.The residence time of the aqueous hydrolysis mixture in the circulationzone upstream of the forward flow port and in the forward flow reactionzone between the forward flow port and the point of dilution was lessthan one second and less than one minute, respectively. After dilution,the finishing reaction stream was fed to the PFR which operatedadiabatically at 82-106° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.02% nitrile and lessthan 0.01% amide. The finished aqueous hydrolyzate product color was 8on a Gardner scale.

EXAMPLE 52

138. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 1 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at61° C., the hydrolysis mixture contained 43% amide and 0.05% nitrile.The residence time of the aqueous hydrolysis mixture in the circulationzone upstream of the forward flow port and in the forward flow reactionzone between the forward flow port and the point of dilution was lessthan one second and less than one minute, respectively. After dilution,the finishing reaction stream was fed to the PFR which operatedadiabatically at 78-103° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.02% amide and thenitrile was not measured. The finished aqueous hydrolyzate product colorwas 5 on a Gardner scale.

EXAMPLE 53

139. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 1 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at61° C., the hydrolysis mixture contained 40% amide and 0.02% nitrile.The residence time of the aqueous hydrolysis mixture in the circulationzone upstream of the forward flow port and in the forward flow reactionzone between the forward flow port and the point of dilution was lessthan one second and less than one minute, respectively. After dilution,the finishing reaction stream was fed to the PFR which operatedadiabatically at 77-102° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.01% amide and 0.05%nitrile. The finished aqueous hydrolyzate product color was 6 on aGardner scale.

EXAMPLE 54

140. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 1 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at64° C., the hydrolysis mixture contained 41% amide and 0.02% nitrile.The residence time of the aqueous hydrolysis mixture in the circulationzone upstream of the forward flow port and in the forward flow reactionzone between the forward flow port and the point of dilution was lessthan one second and less than one minute, respectively. After dilution,the finishing reaction stream was fed to the PFR which operatedadiabatically at 79-103° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.01% amide and 0.03%nitrile. The finished aqueous hydrolyzate product color was notmeasured.

EXAMPLE 55

141. Nitrile, 96% sulfuric acid and water were fed to a CSTR as shown inFIG. 1 giving a 1.0 acid/nitrile molar ratio to form an aqueoushydrolysis mixture. After a residence time of 37 minutes in the CSTR at64° C., the hydrolysis mixture contained 40% amide and 0.01% nitrile.The residence time of the aqueous hydrolysis mixture in the circulationzone upstream of the forward flow port and in the forward flow reactionzone between the forward flow port and the point of dilution was lessthan one second and less than one minute, respectively. After dilution,the finishing reaction stream was fed to the PFR which operatedadiabatically at 79-103° C. and a residence time of 70 minutes. Thefinished aqueous hydrolyzate product contained 0.01% amide and 0.04%nitrile. The finished aqueous hydrolyzate product color was 9 on aGardner scale.

EXAMPLE 56

142. Performance of a continuous hydrolysis system was computersimulated based upon laboratory batch hydrolysis data. 36% hydrochloricacid and nitrile are fed continuously to a CSTR giving a 1.15acid/nitrile molar ratio to form an aqueous hydrolysis mixture. After aresidence time of 60 minutes in the CSTR at 50° C., the hydrolysismixture contains 46% amide and 0.1% nitrile. The nitrile hydrolysisreactor product stream exiting the CSTR is transferred to an amidehydrolysis cascade tower type reactor that is agitated to enhance fluidmixing and suspend ammonium chloride solids in each of the cascadedcompartments. A reactor temperature of 80° C. is provided by jacketssurrounding the reactor shell or by passing the stream through anexternal feed preheater. The residence time of the stream in the amidehydrolysis reactor is 4 hours. The finished aqueous hydrolyzate productcontains 0.04% amide and 0.04% nitrile.

EXAMPLE 57

143. Performance of a continuous hydrolysis system was computersimulated based upon laboratory batch hydrolysis data. 36% hydrochloricacid and nitrile are fed continuously to a CSTR giving a 1.15acid/nitrile molar ratio to form an aqueous hydrolysis mixture. After aresidence time of 60 minutes in the CSTR at 50° C., the hydrolysismixture contains 46% amide and 0.1% nitrile. The nitrile hydrolysisreactor product stream exiting the CSTR is transferred to a second CSTRfor completion of 80-90% of the amide hydrolysis. After a residence timeof 4 hours in the second CSTR, the amide hydrolysis slurry containingammonium chloride is cooled to 50° C. and the ammonium chloride isremoved by centrifuge. The mother liquor from the centrifuge istransferred to a plug flow reactor operated at 80° C. by use ofjacketing or an external preheater. The PFR is not agitated because theammonium chloride is dissolved at the temperatures and concentrationsexisting within the PFR. After a residence time of 2 hours in the PFR,the finished aqueous hydrolyzate product contains 0.04% amide and 0.04%nitrile.

144. While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and have been described herein in detail. Itshould be understood, however, that it is not intended to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

We claim:
 1. A process for the preparation of2-hydroxy-4-methylthiobutanoic acid or a salt thereof comprising:introducing an aqueous mineral acid into a nitrile hydrolysis reactorcomprising a continuous stirred tank reactor; introducing2-hydroxy-4-methylthiobutanenitrile into said nitrile hydrolysisreactor; continuously hydrolyzing 2-hydroxy-4-methylthiobutanenitrilewithin said nitrile hydrolysis reactor to produce a nitrile hydrolysisreactor product stream containing 2-hydroxy-4-methylthiobutanamide;continuously introducing water and said nitrile hydrolysis reactorproduct stream into an amide hydrolysis flow reactor; and continuouslyhydrolyzing 2-hydroxy-4-methylthiobutanamide within said amidehydrolysis flow reactor to produce a finished aqueous hydrolyzateproduct containing 2-hydroxy-4-methylthiobutanoic acid.
 2. The processas set forth in claim 1 wherein sulfuric acid is introduced into saidnitrile hydrolysis reactor in an acid stream having a strength ofbetween about 50% by weight and about 70% by weight sulfuric acid. 3.The process as set forth in claim 1 wherein sulfuric acid is introducedinto said nitrile hydrolysis reactor in an acid stream having a strengthof between about 70% by weight and about 99% by weight sulfuric acid,and said acid stream is continuously introduced to said nitrilehydrolysis reactor concurrently with a water stream to form sulfuricacid having a strength of between about 50% by weight and about 70% byweight on an organic-free basis within said nitrile hydrolysis reactor.4. The process as set forth in claim 1 wherein at least about 90% of2-hydroxy-4-methylthiobutanenitrile is converted to2-hydroxy-4-methylthiobutanamide within said nitrile hydrolysis reactor.5. The process as set forth in claim 1 wherein said aqueous mineral acidis sulfuric acid and the molar ratio of sulfuric acid to2-hydroxy-4-methylthiobutanenitrile introduced into said nitrilehydrolysis reactor is between about 0.6 and about 1.5.
 6. The process asset forth in claim 1 wherein said aqueous mineral acid is sulfuric acidand the molar ratio of sulfuric acid to2-hydroxy-4-methylthiobutanenitrile introduced into said nitrilehydrolysis reactor is between about 0.9 and about 1.2.
 7. The process asset forth in claim 5 wherein the molar ratio of sulfuric acid to2-hydroxy-4-methylthiobutanenitrile introduced into said nitrilehydrolysis reactor is between about 1.0 and about 2.0 during the periodbetween start up of the process until steady state conditions areestablished in said amide hydrolysis flow reactor, and thereafter saidmolar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile isbetween about 0.6 and about 1.5.
 8. The process as set forth in claim 6wherein the molar ratio of sulfuric acid to2-hydroxy-4-methylthiobutanenitrile introduced into said nitrilehydrolysis reactor is between about 1.0 and about 1.5 during the periodbetween start up of the process until steady state conditions areestablished in said amide hydrolysis flow reactor, and thereafter saidmolar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile isbetween about 0.9 and about 1.2.
 9. The process as set forth in claim 1wherein the ratio of the rate of mineral acid flow into said amidehydrolysis flow reactor to the rates of 2-hydroxy-4-methylthiobutanamideand 2-hydroxy-4-methylthiobutanenitrile flow into said amide hydrolysisflow reactor is controlled to provide an excess of at least 5% molarexcess mineral acid than is stoichiometrically equivalent to2-hydroxy-4-methylthiobutanamide and 2-hydroxy-4-methylthiobutanenitrileintroduced into said amide hydrolysis flow reactor.
 10. The process asset forth in claim 9 wherein mineral acid and2-hydroxy-4-methylthiobutanenitrile are introduced into said nitrilehydrolysis reactor at relative rates effective to provide said excess insaid amide hydrolysis flow reactor.
 11. The process as set forth inclaim 1 wherein said finished aqueous hydrolyzate product produced understeady state conditions at the exit of said amide hydrolysis flowreactor comprises at least about 36 wt. % 2-hydroxy-4-methylthiobutanoicacid, at least about 18 wt. % ammonium salt, at least about 20 wt. %water, up to about 0.05 wt. % amide and up to about 0.05 wt. % nitrile.12. The process as set forth in claim 11 wherein said finished aqueoushydrolyzate product produced upon start up of the process comprises upto about 0.05 wt. % amide and up to about 0.05 wt. % nitrile.
 13. Theprocess as set forth in claim 1 wherein said nitrile hydrolysis reactorproduct stream comprises up to about 16 wt. %2-hydroxy-4-methylthiobutanoic acid, up to about 12 wt. % ammonium salt,at least about 6 wt. % water, at least about 30 wt. % amide and up toabout 2 wt. % nitrile.
 14. The process as set forth in claim 1 whereinsaid water and said nitrile hydrolysis reactor product stream are mixedto form a finishing reaction stream such that the hydrolysis of2-hydroxy-4-methylthiobutanamide is substantially completed as saidfinishing reaction stream flows through said amide hydrolysis flowreactor.
 15. The process as set forth in claim 14 wherein said nitrilehydrolysis reactor product stream is diluted with said water before saidnitrile hydrolysis reactor product stream is introduced into said amidehydrolysis flow reactor.
 16. The process as set forth in claim 5 whereinsaid amide hydrolysis flow reactor is a plug flow reactor, and the flowof said finishing reaction stream through said plug flow reactor isturbulent.
 17. The process as set forth in claim 14 wherein said waterstream is heated before being introduced into a mixer for diluting saidnitrile hydrolysis reactor product stream with water to form saidfinishing reaction stream and prevent liquid phase separation.
 18. Theprocess as set forth in claim 14 wherein said amide hydrolysis flowreactor comprises a packed column reactor and said finishing reactionstream flows through said packed column reactor at or above thethreshold velocity of said packed column reactor.
 19. The process as setforth in claim 14 wherein said amide hydrolysis flow reactor comprises apipeline reactor and said finishing reaction stream moves through saidpipeline reactor in turbulent flow.
 20. The process as set forth inclaim 19 wherein said amide hydrolysis flow reactor is operated at aReynolds number greater than about 3,000.
 21. The process as set forthin claim 19 wherein said amide hydrolysis flow reactor is operated at aReynolds number greater than about 5,000.
 22. The process as set forthin claim 1 wherein said amide hydrolysis flow reactor is a plug flowreactor operated at a Peclet number of at least 50, a peak temperatureof about 90 to about 120° C. and a residence time between about 30 andabout 90 minutes.
 23. The process as set forth in claim 1 wherein saidamide hydrolysis flow reactor operates substantially adiabatically. 24.The process as set forth in claim 1 wherein said amide hydrolysis flowreactor operates substantially isothermally.
 25. The process as setforth in claim 1 wherein said amide hydrolysis flow reactor operatesadiabatically and autothermally.
 26. The process as set forth in claim 1further including recovering 2-hydroxy-4-methylthiobutanoic acid or asalt or derivative thereof from said finished aqueous hydrolyzateproduct.
 27. The process as set forth in claim 1 where in2-hydroxy-4-methylthiobutanoic acid is recovered by extracting 2-hydroxy-4-methylthiobutanoic acid from said finished aqueous hydrolyzateproduct.
 28. The process as set forth in claim 1 wherein2-hydroxy-4-methylthiobutanoic acid is recovered by neutralizing saidfinished aqueous hydrolyzate product to form an organic phase containing2-hydroxy-4-methylthiobutanoic acid and an aqueous phase, and separatingsaid organic phase and said aqueous phase to recover2-hydroxy-4-methylthiobutanoic acid.
 29. The process as set forth inclaim 1 wherein vapor emissions from the process are not greater thanabout 0.5 scf per 1000 lbs. product 2-hydroxy-4-methylthiobutanoic acid.30. The process as set forth in claim 29 wherein vapor emissions fromthe process are not greater than about 0.3 scf per 1000 lbs.2-hydroxy-4-methylthiobutanoic acid.
 31. The process as set forth inclaim 1 wherein said aqueous mineral acid, said water stream and saidnitrile hydrolysis reaction product stream are mixed to form thefinishing reaction stream that is introduced into the amide hydrolysisflow reactor.
 32. A process for the preparation of2-hydroxy-4-methylthiobutanoic acid or a salt thereof comprising:introducing mineral acid into a nitrile hydrolysis reactor comprising afirst continuous stirred tank reactor; introducing2-hydroxy-4-methylthiobutanenitrile into said nitrile hydrolysisreactor; continuously hydrolyzing 2-hydroxy-4-methylthiobutanenitrilewithin said nitrile hydrolysis reactor to produce a nitrile hydrolysisreactor product stream containing 2-hydroxy-4-methylthiobutanamide;continuously introducing said nitrile hydrolysis reactor product streamexiting said nitrile hydrolysis reactor and water into a continuousamide hydrolysis reactor comprising a second continuous stirred tankreactor such that a substantial portion of2-hydroxy-4-methylthiobutanamide contained in said nitrile hydrolysisreactor product stream is hydrolyzed in said second continuous stirredtank reactor to form a finishing reaction stream; continuouslyintroducing said finishing reaction stream into an amide hydrolysis flowreactor; and continuously hydrolyzing 2-hydroxy-4-methylthiobutanamidewithin said amide hydrolysis flow reactor to produce a finished aqueoushydrolyzate product containing 2-hydroxy-4-methylthiobutanoic acid. 33.The process as set forth in claim 32 wherein sulfuric acid is introducedinto said nitrile hydrolysis reactor in an acid stream having a strengthof between about 50% by weight and about 70% by weight sulfuric acid.34. The process as set forth in claim 32 wherein sulfuric acid isintroduced into said nitrile hydrolysis reactor in an acid stream havinga strength of between about 70% by weight and about 99% by weightsulfuric acid, and said acid stream is continuously introduced to saidnitrile hydrolysis reactor concurrently with a water stream to formsulfuric acid having a strength of between about 50% by weight and about70% by weight on an organic-free basis within said nitrile hydrolysisreactor.
 35. The process as set forth in claim 32 wherein at least about90% of 2-hydroxy-4-methylthiobutanenitrile is converted to2-hydroxy-4-methylthiobutanamide within said nitrile hydrolysis reactor.36. The process as set forth in claim 32 wherein the molar ratio ofsulfuric acid to 2-hydroxy-4-methylthiobutanenitrile introduced intosaid nitrile hydrolysis reactor is between about 0.6 and about 1.5. 37.The process as set forth in claim 32 wherein the molar ratio of sulfuricacid to 2-hydroxy-4-methylthiobutanenitrile introduced into said nitrilehydrolysis reactor is between about 0.9 and about 1.2.
 38. The processas set forth in claim 32 wherein the molar ratio of sulfuric acid to2-hydroxy-4-methylthiobutanenitrile introduced into said nitrilehydrolysis reactor is between about 1.0 and about 2.0 during the periodbetween start up of the process until steady state conditions areestablished in said amide hydrolysis flow reactor, and thereafter saidmolar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile isbetween about 0.6 and about 1.5.
 39. The process as set forth in claim32 wherein the molar ratio of sulfuric acid to2-hydroxy-4-methylthiobutanenitrile introduced into said nitrilehydrolysis reactor is between about 1.0 and about 1.5 during the periodbetween start up of the process until steady state conditions areestablished in said amide hydrolysis flow reactor, and thereafter saidmolar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile isbetween about 0.9 and about 1.2.
 40. The process as set forth in claim32 wherein the ratio of the rate of mineral acid flow into said amidehydrolysis flow reactor to the rates of 2-hydroxy-4-methylthiobutanamideand 2-hydroxy-4-methylthiobutanenitrile flow into said amide hydrolysisflow reactor is controlled to provide an excess of at least 5% molarexcess mineral acid than is stoichiometrically equivalent to2-hydroxy-4-methylthiobutanamide and 2-hydroxy-4-methylthiobutanenitrileintroduced into said amide hydrolysis flow reactor.
 41. The process asset forth in claim 40 wherein mineral acid and2-hydroxy-4-methylthiobutanenitrile are introduced into said nitrilehydrolysis reactor at relative rates effective to provide said excess insaid amide hydrolysis flow reactor.
 42. The process as set forth inclaim 32 wherein said finished aqueous hydrolyzate product producedunder steady state conditions at the exit of said amide hydrolysis flowreactor comprises at least about 36 wt. % 2-hydroxy-4-methylthiobutanoicacid, at least about 18 wt. % ammonium salt, at least about 20 wt. %water, up to about 0.05 wt. % amide and up to about 0.05 wt. % nitrile.43. The process as set forth in claim 42 wherein said finished aqueoushydrolyzate product produced upon start up of the process comprises upto about 0.05 wt. % amide and up to about 0.05 wt. % nitrile.
 44. Theprocess as set forth in claim 32 wherein said nitrile hydrolysis reactorproduct comprises up to about 16 wt. % 2-hydroxy-4-methylthiobutanoicacid, up to about 12 wt. % ammonium salt, at least about 6 wt. % water,at least about 30 wt. % amide and up to about 2 wt. % nitrile.
 45. Theprocess as set forth in claim 32 wherein said amide hydrolysis flowreactor is a plug flow reactor, and the flow of said finishing reactionstream through said plug flow reactor is turbulent.
 46. The process asset forth in claim 32 wherein said nitrile hydrolysis reactor productstream is diluted with said water before said nitrile hydrolysis reactorproduct stream is introduced into said continuous amide hydrolysisreactor.
 47. The process as set forth in claim 32 wherein said amidehydrolysis flow reactor comprises a packed column reactor and saidfinishing reaction stream flows through said packed column reactor at orabove the threshold velocity of said packed column reactor.
 48. Theprocess as set forth in claim 32 wherein said amide hydrolysis flowreactor comprises a pipeline reactor and said finishing reaction streammoves through said pipeline reactor in turbulent flow.
 49. The processas set forth in claim 48 wherein said amide hydrolysis flow reactor isoperated at a Reynolds number greater than about 3,000.
 50. The processas set forth in claim 48 wherein said amide hydrolysis flow reactor isoperated at a Reynolds number greater than about 5,000.
 51. The processas set forth in claim 32 wherein said amide hydrolysis flow reactor is aplug flow reactor operated at a Peclet number of at least 50, a peaktemperature of about 90 to about 120° C. and a residence time betweenabout 30 and about 90 minutes.
 52. The process as set forth in claim 32wherein said amide hydrolysis flow reactor operates substantiallyadiabatically.
 53. The process as set forth in claim 32 wherein saidamide hydrolysis flow reactor operates isothermally.
 54. The process asset forth in claim 32 wherein said amide hydrolysis flow reactoroperates adiabatically and autothermally.
 55. The process as set forthin claim 32 further including recovering 2-hydroxy-4-methylthiobutanoicacid or a salt or derivative thereof from said finished aqueoushydrolyzate product.
 56. The process as set forth in claim 32 wherein2-hydroxy-4-methylthiobutanoic acid is recovered by extracting2-hydroxy-4-methylthiobutanoic acid from said finished aqueoushydrolyzate product.
 57. The process as set forth in claim 32 wherein2-hydroxy-4-methylthiobutanoic acid is recovered by neutralizing saidfinished aqueous hydrolyzate product to form an organic phase containing2-hydroxy-4-methylthiobutanoic acid and an aqueous phase, and separatingsaid organic phase and said aqueous phase to recover2-hydroxy-4-methylthiobutanoic acid.
 58. The process as set forth inclaim 32 wherein vapor emissions from the process are not greater thanabout 0.5 scf per 1000 lbs. product 2-hydroxy-4-methylthiobutanoic acid.59. The process as set forth in claim 58 wherein vapor emissions fromthe process are not greater than about 0.3 scf per 1000 lbs.2-hydroxy-4-methylthiobutanoic acid.
 60. The process as set forth inclaim 32 wherein at least about 80% of 2-hydroxy-4-methylthiobutanamideformed in said nitrile hydrolysis reactor is converted to2-hydroxy-4-methylthiobutanoic acid within said continuous amidehydrolysis reactor.
 61. The process as set forth in claim 36 whereinsaid continuous amide hydrolysis reactor is operated at a temperatureranging from about 70° C. to about 120° C.
 62. The process as set forthin claim 32 wherein said aqueous mineral acid, said water stream andsaid nitrile hydrolysis reaction product stream are mixed to form anamide hydrolysis mixture that is introduced into the continuous amidehydrolysis reactor.
 63. A process for the preparation of2-hydroxy-4-methylthiobutanoic acid or a salt thereof comprising:introducing an aqueous mineral acid and2-hydroxy-4-methylthiobutanenitrile into said first reactor;continuously hydrolyzing 2-hydroxy-4-methylthiobutanenitrile within saidfirst reactor to produce a nitrile hydrolysis reactor product streamcontaining 2-hydroxy-4-methylthiobutanamide; continuously introducingwater, aqueous mineral acid, and said nitrile hydrolysis reactor productstream into a second reactor; and continuously hydrolyzing2-hydroxy-4-methylthiobutanamide within said second reactor to produce afinished aqueous hydrolyzate product containing2-hydroxy-4-methylthiobutanoic acid.
 64. A process as set forth in claim63 wherein the molar ratio of mineral acid to2-hydroxy-4-methylthiobutanenitrile added to said first reactor isbetween about 0.6 and about 1.5, and the overall molar ratio of mineralacid to 2-hydroxy-4-methylthiobutanenitrile is between about 0.7 andabout 1.5.
 65. A process as set forth in claim 64 wherein the molarratio of mineral acid to 2-hydroxy-4-methylthiobutanenitrile added tosaid first reactor is between about 0.8 and about 1.2, and the overallmolar ratio of mineral acid to 2-hydroxy-4-methylthiobutanenitrile isbetween about 0.9 and about 1.2.
 66. A process as set forth in claim 63wherein the molar ratio of mineral acid to2-hydroxy-4-methylthiobutanenitrile added to said nitrile hydrolysisreactor is between about 0.5 and about 0.95, and the overall molar ratioof mineral acid to 2-hydroxy-4-methylthiobutanenitrile is between about0.6 and about 0.95.
 67. A process as set forth in claim 66 wherein themolar ratio of mineral acid to 2-hydroxy-4-methylthiobutanenitrile addedto said nitrile hydrolysis reactor is between about 0.8 and about 0.95,and the overall molar ratio of mineral acid to2-hydroxy-4-methylthiobutanenitrile is between about 0.85 and about0.95.
 68. The process as set forth in claim 63 wherein said firstreactor is a nitrile hydrolysis reactor comprising a first continuousstirred tank reactor, 2-hydroxy-4-methylthiobutanenitrile iscontinuously hydrolyzed within said nitrile hydrolysis reactor, saidnitrile hydrolysis product stream is introduced into said second reactorwhich is an amide hydrolysis flow reactor, and2-hydroxy-4-methylthiobutanamide is continuously hydrolyzed within saidamide hydrolysis flow reactor.
 69. The process as set forth in claim 63wherein said first reactor is a nitrile hydrolysis reactor comprising afirst continuous stirred tank reactor,2-hydroxy-4-methylthiobutanenitrile is continuously hydrolyzed withinsaid nitrile hydrolysis reactor, said nitrile hydrolysis reactor productstream is introduced into a continuous amide hydrolysis reactorcomprising a second continuous stirred tank reactor,2-hydroxy-4-methylthiobutanamide is continuously hydrolyzed within saidsecond continuous stirred tank reactor to form a finishing reactionstream, said finishing reaction stream is introduced into said secondreactor which is an amide hydrolysis flow reactor, and said hydrolysisof 2-hydroxy-4-methylthiobutanamide is substantially completed as saidfinishing reaction stream flows through said amide hydrolysis flowreactor.
 70. A process for the preparation of2-hydroxy-4-methylthiobutanoic acid or a salt thereof comprising:concurrently introducing 2-hydroxy-4-methylthiobutanenitrile,concentrated sulfuric acid stream having a strength of between about 70%by weight and about 99% by weight, and water into a vessel in which2-hydroxy-4-methylthiobutanenitrile is hydrolyzed; hydrolyzing2-hydroxy-4-methylthiobutanenitrile within said vessel to produce anitrile hydrolysis product stream containing2-hydroxy-4-methylthiobutanamide; and hydrolyzing2-hydroxy-4-methylthiobutanamide to produce a finished aqueoushydrolyzate product containing 2-hydroxy-4-methylthiobutanoic acid. 71.The process as set forth in claim 70 wherein said vessel is a nitrilehydrolysis reactor comprising a first continuous stirred tank reactor,2-hydroxy-4-methylthiobutanenitrile is continuously hydrolyzed withinsaid nitrile hydrolysis reactor, said nitrile hydrolysis product streamis introduced into an amide hydrolysis flow reactor, and2-hydroxy-4-methylthiobutanamide is continuously hydrolyzed within saidamide hydrolysis flow reactor.
 72. The process as set forth in claim 70wherein said vessel is a nitrile hydrolysis reactor comprising a firstcontinuous stirred tank reactor, 2-hydroxy-4-methylthiobutanenitrile iscontinuously hydrolyzed within said nitrile hydrolysis reactor, saidnitrile hydrolysis reactor product stream is introduced into acontinuous amide hydrolysis reactor comprising a second continuousstirred tank reactor, 2-hydroxy-4-methylthiobutanamide is continuouslyhydrolyzed within said second continuous stirred tank reactor to form afinishing reaction stream, said finishing reaction stream is introducedinto an amide hydrolysis flow reactor, and said hydrolysis of2-hydroxy-4-methylthiobutanamide is substantially completed as saidfinishing reaction stream flows through said amide hydrolysis flowreactor.
 73. The process as set forth in claim 70 further includingrecovering 2-hydroxy-4-methylthiobutanoic acid or a salt or derivativethereof from said finished aqueous hydrolyzate product.
 74. The processas set forth in claim 70 wherein 2-hydroxy-4-methylthiobutanoic acid isrecovered by extracting 2-hydroxy-4-methylthiobutanoic acid from saidfinished aqueous hydrolyzate product.
 75. The process as set forth inclaim 70 wherein vapor emissions from the process are not greater thanabout 0.5 scf per 1000 lbs. product 2-hydroxy-4-methylthiobutanoic acid.76. An apparatus for use in a process for the preparation of2-hydroxy-4-methylthiobutanoic acid, comprising a first continuousstirred tank reactor for the continuous hydrolysis of2-hydroxy-4-methylthiobutanenitrile in the presence of an aqueousmineral acid to produce an aqueous hydrolysis mixture containing2-hydroxy-4-methylthiobutanamide, and an amide hydrolysis flow reactorfor the continuous hydrolysis of 2-hydroxy-4-methylthiobutanamide withsaid aqueous mineral acid to produce a finished aqueous hydrolyzateproduct containing 2-hydroxy-4-methylthiobutanoic acid.
 77. Theapparatus as set forth in claim 76 further including a second continuousstirred tank reactor for receiving water and said aqueous hydrolysismixture exiting said first continuous stirred tank reactor, such that asubstantial portion of 2-hydroxy-4-methylthiobutanamide contained insaid aqueous hydrolysis mixture is hydrolyzed in said second continuousstirred tank reactor to form a finishing reaction stream, and thehydrolysis of 2-hydroxy-4-methylthiobutanamide is substantiallycompleted as said finishing reaction stream flows through said amidehydrolysis flow reactor.
 78. The apparatus as set forth in claim 76further including a circulating line for circulating said aqueoushydrolysis solution from an exit of said first continuous stirred tankreactor through said circulating line and back to said first continuousstirred tank reactor, a forward flow port in said circulating line forremoving a portion of said aqueous hydrolysis mixture to form a nitrilehydrolysis reactor product stream, and a transfer line for transportingsaid nitrile hydrolysis reactor product stream to a point of dilution,said circulating line and said transfer line providing additionalresidence time for substantially extinguishing residual2-hydroxy-4-methylthiobutanenitrile prior to dilution of said nitrilehydrolysis reactor product stream.
 79. The apparatus as set forth inclaim 76 further including a mixer for mixing a water stream and saidnitrile hydrolysis reactor product stream exiting said first continuousstirred tank reactor to form a finishing reaction stream, anddischarging said finishing reaction stream to said amide hydrolysis flowreactor such that the hydrolysis of 2-hydroxy-4-methylthiobutanamide issubstantially completed as said finishing reaction stream flows throughsaid amide hydrolysis flow reactor.
 80. The apparatus as set forth inclaim 76 including a mixer for mixing said aqueous mineral acid, saidwater stream and said nitrile hydrolysis reaction product stream to formthe finishing reaction stream that is introduced into the amidehydrolysis flow reactor.
 81. The apparatus as set forth in claim 76wherein said amide hydrolysis flow reactor is insulated for adiabaticoperation.
 82. The apparatus as set forth in claim 76 wherein saidnitrile hydrolysis reactor comprises an inlet for2-hydroxy-4-methylthiobutanenitrile, an inlet for concentrated mineralacid, an inlet for water, and means within said reactor for mixing2-hydroxy-4-methylthiobutanenitrile, concentrated mineral acid and waterin proportions suited for hydrolysis of2-hydroxy-4-methylthiobutanenitrile to 2-hydroxy-4-methylthiobutanamide.83. The apparatus as set forth in claim 82 further including means forremoving heat generated by dilution of mineral acid and reaction of2-hydroxy-4-methylthiobutanenitrile and water in order to maintain areaction temperature for hydrolysis of2-hydroxy-4-methylthiobutanenitrile.
 84. An apparatus for use in aprocess for the preparation of 2-hydroxy-4-methylthiobutanoic acid,comprising a first back-mixed reactor for the continuous hydrolysis of2-hydroxy-4-methylthiobutanenitrile in the presence of an aqueousmineral acid to produce an aqueous hydrolysis mixture containing2-hydroxy-4-methylthiobutanamide, and an amide hydrolysis flow reactorfor the continuous hydrolysis of 2-hydroxy-4-methylthiobutanamide withsaid aqueous mineral acid to produce a finished aqueous hydrolyzateproduct containing 2-hydroxy-4-methylthiobutanoic acid.
 85. Theapparatus as set forth in claim 84 further including a second back-mixedreactor for receiving water and said aqueous hydrolysis mixture exitingsaid first back-mixed reactor, such that a substantial portion of2-hydroxy-4-methylthiobutanamide contained in said aqueous hydrolysismixture is hydrolyzed in said second back-mixed reactor to form afinishing reaction stream, and the hydrolysis of2-hydroxy-4-methylthiobutanamide is substantially completed as saidfinishing reaction stream flows through said amide hydrolysis flowreactor.
 86. The apparatus as set forth in claim 84 including acirculating line for circulating said aqueous hydrolysis solution froman exit of said first back-mixed reactor through said circulating lineand back to said first back-mixed reactor, a forward flow port in saidcirculating line for removing a portion of said aqueous hydrolysismixture to form a nitrile hydrolysis reactor product stream, and atransfer line for transporting said nitrile hydrolysis reactor productstream to a point of dilution, said circulating line and said transferline providing additional residence time for substantially extinguishingresidual 2-hydroxy-4-methylthiobutanenitrile prior to dilution of saidnitrile hydrolysis reactor product stream.
 87. The apparatus as setforth in claim 84 further including a mixer for mixing a water streamand said nitrile hydrolysis reactor product stream exiting said firstback-mixed reactor to form a finishing reaction stream, and dischargingsaid finishing reaction stream to said amide hydrolysis flow reactorsuch that the hydrolysis of 2-hydroxy-4-methylthiobutanamide issubstantially completed as said finishing reaction stream flows throughsaid amide hydrolysis flow reactor.
 88. The apparatus as set forth inclaim 84 including a mixer for mixing said aqueous mineral acid, saidwater stream and said nitrile hydrolysis reaction product stream to formthe finishing reaction stream that is introduced into the amidehydrolysis flow reactor.
 89. The apparatus as set forth in claim 84wherein said amide hydrolysis flow reactor is insulated for adiabaticoperation.
 90. The apparatus as set forth in claim 84 wherein said firstback-mixed reactor comprises an inlet for2-hydroxy-4-methylthiobutanenitrile, an inlet for concentrated mineralacid, an inlet for water, and means within said reactor for mixing2-hydroxy-4-methylthiobutanenitrile, concentrated mineral acid and waterin proportions suited for hydrolysis of2-hydroxy-4-methylthiobutanenitrile to 2-hydroxy-4-methylthiobutanamide.91. The apparatus as set forth in claim 86 wherein said circulating linecomprises an inlet for concentrated mineral acid, said first back-mixedreactor comprises an inlet for 2-hydroxy-4-methylthiobutanenitrile andan inlet for water, and means within said reactor for mixing2-hydroxy-4-methylthiobutanenitrile, concentrated mineral acid and waterin proportions suited for hydrolysis of2-hydroxy-4-methylthiobutanenitrile to 2-hydroxy-4-methylthiobutanamide.92. The apparatus as set forth in claim 90 further including means forremoving heat generated by dilution of mineral acid and reaction of2-hydroxy-4-methylthiobutanenitrile and water in order to maintain areaction temperature for hydrolysis of2-hydroxy-4-methylthiobutanenitrile.
 93. A process for the preparationof 2-hydroxy-4-methylthiobutanoic acid or a salt thereof comprising:introducing 2-hydroxy-4-methylthiobutanenitrile and an aqueous mineralacid into an aqueous hydrolysis mixture comprising2-hydroxy-4-methylthiobutanamide, mineral acid, and unreacted2-hydroxy-4-methylthiobutanenitrile; hydrolyzing2-hydroxy-4-methylthiobutanenitrile in said hydrolysis mixture in acontinuous nitrile hydrolysis reactor comprising a back-mixed reactionzone and a circulation zone in fluid flow communication with saidback-mixed reaction zone, said circulation zone comprising a circulatingline; continuously circulating said aqueous hydrolysis mixture in acirculating stream that is withdrawn from said back-mixed reaction zone,passed through said circulation zone and returned to said back-mixedreaction zone, said circulating stream as withdrawn from said back-mixedreaction zone containing unreacted 2-hydroxy-4-methylthiobutanenitrile;removing a portion of said circulating aqueous hydrolysis mixture as anitrile hydrolysis reactor product stream from a forward flow port insaid circulation zone and transferring said nitrile hydrolysis reactorproduct stream to an amide hydrolysis flow reactor; diluting saidnitrile hydrolysis reactor product stream with water at a pointdownstream of said forward flow port to provide a finishing reactionstream; hydrolyzing 2-hydroxy-4-methylthiobutanamide contained in saidfinishing reaction stream in said amide hydrolysis flow reactor toproduce a finished aqueous hydrolyzate product containing2-hydroxy-4-methylthiobutanoic acid; the sum of the residence time ofsaid circulating stream in said circulation zone upstream of saidforward flow port and the residence time of said nitrile hydrolysisreactor product stream downstream of said forward flow port prior todilution being sufficient to substantially extinguish residual2-hydroxy-4-methylthiobutanenitrile prior to the dilution of saidnitrile hydrolysis reactor product stream.
 94. A process as set forth inclaim 93 wherein the residual 2-hydroxy-4-methylthiobutanenitrile insaid nitrile hydrolysis reactor product stream at said point of dilutionis not greater than about 0.01 wt % based on the sum of the2-hydroxy-4-methylthiobutanamide and 2-hydroxy-4-methylthiobutanoic acidcontained in said product stream.
 95. A process as set forth in claim 93wherein the residual 2-hydroxy-4-methylthiobutanenitrile in said aqueoushydrolysis mixture exiting said nitrile hydrolysis reactor is at leastabout 0.05 wt % based on the sum of the 2-hydroxy-4-methylthiobutanamideand 2-hydroxy-4-methylthiobutanoic acid contained in said aqueoushydrolysis mixture.
 96. A process as set forth in claim 93 wherein theflow regime in said circulating line is substantially turbulent, thetemperature of said circulating stream is at least 50° C. throughoutsaid circulation zone, and the residence time of said circulating streamin said circulation zone upstream of said forward flow port is at leastabout 3 seconds.
 97. A process as set forth in claim 93 wherein betweensaid forward flow port and said point of dilution the temperature ofsaid nitrile hydrolysis reactor product stream is at least 50° C. andthe residence time is at least about 30 seconds.
 98. A process as setforth in claim 93 wherein the flow regimes and residence times are suchas to provide the equivalent of at least one sequential back-mixedreaction zone in a nitrile extinction reaction region comprising theportion of said circulation zone upstream of said forward flow port anda forward flow reaction zone within which said nitrile reactor productstream flows between said forward flow port and said point of dilution.99. A process as set forth in claim 93 wherein an external heatexchanger is positioned in said circulating line.
 100. A process as setforth in claim 99 wherein said forward flow port is positioned in saidcirculating line upstream of said external heat exchanger.
 101. Aprocess as set forth in claim 99 wherein said aqueous mineral acid isintroduced into said aqueous hydrolysis mixture at a port positioned insaid circulating line downstream of said external heat exchanger.
 102. Aprocess as set forth in claim 93 wherein said nitrile hydrolysis reactorproduct stream is transferred to said point of dilution via a verticaldowncomer line.
 103. A process as set forth in claim 93 wherein saidaqueous mineral acid, said water stream and said nitrile hydrolysisreaction product stream are mixed to form the finishing reaction streamthat is introduced into the amide hydrolysis flow reactor.
 104. Aprocess as set forth in claim 103 wherein the molar ratio of mineralacid to 2-hydroxy-4-methylthiobutanenitrile added to said nitrilehydrolysis reactor being between about 0.6 and about 1.5, and theoverall molar ratio of mineral acid to2-hydroxy-4-methylthiobutanenitrile being between about 0.7 and about1.5.
 105. A process as set forth in claim 93 wherein the flow regime ina forward flow reaction zone within which said nitrile reactor productstream flows between said forward flow port and said point of dilutionis laminar, and the flow regime and residence time are such as toprovide the equivalent of at least one sequential back-mixed reactionzone in said forward flow reaction zone.
 106. A process as set forth inclaim 105 wherein the flow regime and residence time are such as toprovide the equivalent of between about 2 and about 3 sequentialback-mixed reaction zones in said forward flow reaction zone.
 107. Aprocess as set forth in claim 93 wherein the flow regime in a forwardflow reaction zone within which said nitrile reactor product streamflows between said forward flow port and said point of dilution isturbulent, and the flow regime and residence time are such as to providethe equivalent of at least one sequential back-mixed reaction zone insaid forward flow reaction zone.
 108. A process as set forth in claim 97wherein the flow regime in said zone between an inlet of said amidehydrolysis flow reactor and said point of dilution is turbulent.
 109. Aprocess as set forth in claim 93 wherein said finished aqueoushydrolyzate product comprises up to about 0.05 wt. % amide.
 110. Aprocess as set forth in claim 93 wherein the flow regime and residencetime are such as to provide the equivalent of at least two sequentialback-mixed reaction zones in said amide hydrolysis flow reactor.
 111. Aprocess as set forth in claim 93 wherein the flow regime and residencetime are such as to provide the equivalent of at least 35 sequentialback-mixed reaction zones in said amide hydrolysis flow reactor.
 112. Aprocess as set forth in claim 93 wherein the temperature within saidamide hydrolysis flow reactor is controlled to maintain said finishedaqueous hydrolyzate product at a color of less than about 10 on theGardner scale.
 113. A process for the preparation of2-hydroxy-4-methylthiobutanoic acid or a salt thereof comprising:introducing 2-hydroxy-4-methylthiobutanenitrile and an aqueous mineralacid into an aqueous hydrolysis mixture comprising2-hydroxy-4-methylthiobutanamide, mineral acid, and unreacted2-hydroxy-4-methylthiobutanenitrile; hydrolyzing2-hydroxy-4-methylthiobutanenitrile in said hydrolysis mixture in acontinuous nitrile hydrolysis reactor comprising a back-mixed reactionzone and a circulation zone in fluid flow communication with saidback-mixed reaction zone, said circulation zone comprising a circulatingline; continuously circulating said aqueous hydrolysis mixture in acirculating stream that is withdrawn from said back-mixed reaction zone,passed through said circulation zone and returned to said back-mixedreaction zone, said circulating stream as withdrawn from said back-mixedreaction zone containing unreacted 2-hydroxy-4-methylthiobutanenitrile;removing a portion of said circulating aqueous hydrolysis mixture as anitrile hydrolysis reactor product stream from a forward flow port insaid circulation zone and transferring said nitrile hydrolysis reactorproduct stream to a continuous amide hydrolysis reactor; diluting saidnitrile hydrolysis reactor product stream with water at a pointdownstream of said forward flow port to provide an amide hydrolysismixture; feeding either said amide hydrolysis mixture, or said nitrilehydrolysis reactor product stream and water, to a continuous amidehydrolysis reactor in which a substantial portion of2-hydroxy-4-methylthiobutanamide contained in said amide hydrolysismixture is hydrolyzed to form a finishing reaction stream; transferringsaid finishing reaction stream to an amide hydrolysis flow reactor;hydrolyzing 2-hydroxy-4-methylthiobutanamide contained in said finishingreaction stream in said amide hydrolysis flow reactor to produce afinished aqueous hydrolyzate product containing2-hydroxy-4-methylthiobutanoic acid; the sum of the residence time ofsaid circulating stream in said circulation zone upstream of saidforward flow port and the residence time of said nitrile hydrolysisreactor product stream downstream of said forward flow point prior todilution being sufficient to substantially extinguish residual2-hydroxy-4-methylthiobutanenitrile prior to the dilution of saidnitrile hydrolysis reactor product stream.
 114. A process as set forthin claim 113 wherein said nitrile hydrolysis reactor product stream isdiluted with water in said continuous amide hydrolysis reactor.
 115. Aprocess as set forth in claim 113 wherein the residual2-hydroxy-4-methylthiobutanenitrile in said nitrile hydrolysis reactorproduct stream at said point of dilution is not greater than about 0.01wt % based on the sum of the 2-hydroxy-4-methylthiobutanamide and2-hydroxy-4-methylthiobutanoic acid contained in said product stream.116. A process as set forth in claim 113 wherein the residual2-hydroxy-4-methylthiobutanenitrile in said aqueous hydrolysis mixtureexiting said nitrile hydrolysis reactor is at least about 0.05 wt %based on the sum of the 2-hydroxy-4-methylthiobutanamide and2-hydroxy-4-methylthiobutanoic acid contained in said aqueous hydrolysismixture.
 117. A process as set forth in claim 113 wherein the flowregime in said circulating line is substantially turbulent, thetemperature of said circulating stream is at least about 50° C.throughout said circulation zone, and the residence time of saidcirculating stream in said circulation zone upstream of said forwardflow port is at least about 3 seconds.
 118. A process as set forth inclaim 113 wherein between said forward flow port and said point ofdilution the temperature of said nitrile hydrolysis reactor productstream is at least about 50° C. and the residence time is at least about30 seconds.
 119. A process as set forth in claim 113 wherein the flowregimes and residence times are such as to provide the equivalent of atleast one sequential back-mixed reaction zone in a nitrile extinctionreaction region comprising the portion of said circulation zone upstreamof said forward flow port and a forward flow reaction zone within whichsaid nitrile reactor product stream flows between said forward flow portand said point of dilution or said continuous amide hydrolysis reactor.120. A process as set forth in claim 113 wherein an external heatexchanger is positioned in said circulating line.
 121. A process as setforth in claim 120 wherein said forward flow port is positioned in saidcirculating line upstream of said external heat exchanger.
 122. Aprocess as set forth in claim 120 wherein said aqueous mineral acid isintroduced into said aqueous hydrolysis mixture at a port positioned insaid circulating line downstream of said external heat exchanger.
 123. Aprocess as set forth in claim 113 wherein said nitrile hydrolysisreactor product stream is transferred to said point of dilution via avertical downcomer line.
 124. A process as set forth in claim 113wherein said aqueous mineral acid, said water stream and said nitrilehydrolysis reaction product stream are mixed to form the finishingreaction stream that is introduced into the amide hydrolysis flowreactor.
 125. A process as set forth in claim 124 wherein the molarratio of mineral acid to 2-hydroxy-4-methylthiobutanenitrile added tosaid nitrile hydrolysis reactor being between about 0.6 and about 1.5,and the overall molar ratio of mineral acid to2-hydroxy-4-methylthiobutanenitrile being between about 0.7 and about1.5.
 126. A process as set forth in claim 113 wherein the flow regime ina forward flow reaction zone within which said nitrile reactor productstream flows between said forward flow port and said point of dilutionor said continuous amide hydrolysis reactor is laminar, and the flowregime and residence time are such as to provide the equivalent of atleast one sequential back-mixed reaction zone in said forward flowreaction zone.
 127. A process as set forth in claim 126 wherein the flowregime and residence time are such as to provide the equivalent ofbetween about 2 and about 3 sequential back-mixed reaction zones in saidforward flow reaction zone.
 128. A process as set forth in claim 113wherein the flow regime in a forward flow reaction zone within whichsaid nitrile reactor product stream flows between said forward flow portand said point of dilution is turbulent, and the flow regime andresidence time are such as to provide the equivalent of at least onesequential back-mixed reaction zone in said forward flow reaction zone.129. A process as set forth in claim 113 wherein said finished aqueoushydrolyzate product comprises up to about 0.05 wt. % amide.
 130. Aprocess as set forth in claim 113 wherein the flow regime and residencetime are such as to provide the equivalent of at least two sequentialback-mixed reaction zones in said amide hydrolysis flow reactor.
 131. Aprocess as set forth in claim 113 wherein the flow regime and residencetime are such as to provide the equivalent of at least 35 sequentialback-mixed reaction zones in said amide hydrolysis flow reactor.
 132. Aprocess as set forth in claim 113 wherein the temperature within saidamide hydrolysis flow reactor is controlled to maintain said finishedaqueous hydrolyzate product at a color of less than about 10 on theGardner scale.
 133. A process for the preparation of2-hydroxy-4-methylthiobutanoic acid or a salt thereof comprising:introducing 2-hydroxy-4-methylthiobutanenitrile and an aqueous mineralacid into an aqueous hydrolysis mixture comprising2-hydroxy-4-methylthiobutanamide, mineral acid, and unreacted2-hydroxy-4-methylthiobutanenitrile; hydrolyzing2-hydroxy-4-methylthiobutanenitrile in said hydrolysis mixture in acontinuous nitrile hydrolysis reactor comprising a back-mixed reactionzone and a circulation zone in fluid flow communication with saidback-mixed reaction zone, said circulation zone comprising a circulatingline; continuously circulating said aqueous hydrolysis mixture in acirculating stream that is withdrawn from said back-mixed reaction zone,passed through said circulation zone and returned to said back-mixedreaction zone, said circulating stream as withdrawn from said back-mixedreaction zone containing unreacted 2-hydroxy-4-methylthiobutanenitrile;removing a portion of said circulating aqueous hydrolysis mixture as anitrile hydrolysis reactor product stream from a forward flow port insaid circulation zone and transferring said nitrile hydrolysis reactorproduct stream to an amide hydrolysis flow reactor; diluting saidnitrile hydrolysis reactor product stream with water at a pointdownstream of said forward flow port to provide a finishing reactionstream; hydrolyzing 2-hydroxy-4-methylthiobutanamide contained in saidfinishing reaction stream in said amide hydrolysis flow reactor toproduce a finished aqueous hydrolyzate product containing2-hydroxy-4-methylthiobutanoic acid; the sum of the residence time ofsaid circulating stream in said circulation zone upstream of saidforward flow port and the residence time of said nitrile hydrolysisreactor product stream downstream of said forward flow port prior todilution being at least about 20 seconds.
 134. A process as set forth inclaim 133 wherein the sum of the residence times is between about 30seconds and about 5 minutes.
 135. A process as set forth in claim 133wherein the flow regime in said circulating line is substantiallyturbulent, and the residence time of said circulating stream in saidcirculation zone upstream of said forward flow port is at least about 3seconds.
 136. A process as set forth in claim 133 wherein between saidforward flow port and said point of dilution the residence time is atleast about 30 seconds.
 137. A process as set forth in claim 133 whereinthe flow regimes and residence times are such as to provide theequivalent of at least one sequential back-mixed reaction zone in anitrile extinction reaction region comprising the portion of saidcirculation zone upstream of said forward flow port and a forward flowreaction zone within which said nitrile reactor product stream flowsbetween said forward flow port and said point of dilution.
 138. Aprocess as set forth in claim 133 wherein an external heat exchanger ispositioned in said circulating line.
 139. A process as set forth inclaim 138 wherein said forward flow port is positioned in saidcirculating line upstream of said external heat exchanger.
 140. Aprocess as set forth in claim 138 wherein said aqueous mineral acid isintroduced into said aqueous hydrolysis mixture at a port positioned insaid circulating line downstream of said external heat exchanger.
 141. Aprocess as set forth in claim 133 wherein said nitrile hydrolysisreactor product stream is transferred to said point of dilution via avertical downcomer line.
 142. A process as set forth in claim 133wherein said aqueous mineral acid, said water stream and said nitrilehydrolysis reaction product stream are mixed to form the finishingreaction stream that is introduced into the amide hydrolysis flowreactor.
 143. A process as set forth in claim 142 wherein the molarratio of mineral acid to 2-hydroxy-4-methylthiobutanenitrile added tosaid nitrile hydrolysis reactor being between about 0.6 and about 1.5,and the overall molar ratio of mineral acid to2-hydroxy-4-methylthiobutanenitrile added to said amide hydrolysis flowreactor being between about 0.7 and about 1.5.
 144. A process as setforth in claim 133 wherein the flow regime in a forward flow reactionzone within which said nitrile reactor product stream flows between saidforward flow port and said point of dilution is laminar, and the flowregime and residence time are such as to provide the equivalent of atleast one sequential back-mixed reaction zone in said forward flowreaction zone.
 145. A process as set forth in claim 144 wherein the flowregime and residence time are such as to provide the equivalent ofbetween about 2 and about 3 sequential back-mixed reaction zones in saidforward flow reaction zone.
 146. A process as set forth in claim 133wherein the flow regime in a forward flow reaction zone within whichsaid nitrile reactor product stream flows between said forward flow portand said point of dilution is turbulent, and the flow regime andresidence time are such as to provide the equivalent of at least onesequential back-mixed reaction zone in said forward flow reaction zone.147. A process as set forth in claim 133 wherein said finished aqueoushydrolyzate product comprises up to about 0.05 wt. % amide.
 148. Aprocess as set forth in claim 133 wherein the flow regime and residencetime are such as to provide the equivalent of at least two sequentialback-mixed reaction zones in said amide hydrolysis flow reactor.
 149. Aprocess as set forth in claim 133 wherein the flow regime and residencetime are such as to provide the equivalent of at least 35 sequentialback-mixed reaction zones in said amide hydrolysis flow reactor.
 150. Anaqueous hydrolyzate composition used in preparing concentrated aqueoussolutions of 2-hydroxy-4-methylthiobutanoic acid as a methioninesupplement, said composition comprising at least about 36 wt. %2-hydroxy-4-methylthiobutanoic acid, at least about 18 wt. % ammoniumsalt, at least about 15 wt. % water, up to about 0.05 wt. % amide and upto about 0.05 wt. % nitrile, said composition having a color of not morethan about 10 on the Gardner scale.
 151. A composition as set forth inclaim 150 including at least about 30 wt. % ammonium salt and at leastabout 25 wt. % water.
 152. A composition as set forth in claim 150wherein the color of said composition is between about 5 and about 10 onthe Gardner scale.
 153. An aqueous hydrolyzate composition used inpreparing concentrated aqueous solutions of2-hydroxy-4-methylthiobutanoic acid as a methionine supplement, saidaqueous hydrolyzate composition having a color of not more than 10 onthe Gardner scale.